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FRAGMENTS    OF 
SCIENCE 


BY 


JOHN   TYNDALL,   F.R.S. 


PART  ONE 


NEW  YORK 
P.    F.    COLLIER    &    SON 

M  C  M  V 

5 


mas' 

V.I 


SCIENCE 


PREFACE   TO   THE   SIXTH    EDITION 


To  AVOID  unwieldiness  of  bulk  this  edition  of  the 
*' Fragments"  is  published  in  two  volumes,  instead  of, 
as  heretofore,  in  one. 

The  first  volume  deals  almost  exclusively  with  the  laws 
and  phenomena  of  matter.  The  second  trenches  upon 
questions  in  which  the  phenomena  of  matter  interlace 
more   or   less   with   those   of   mind. 

New  Essays  have  been  added,  while  old  ones  have 
been  revised,  and  in  part  recast.  To  be  clear,  without 
being  superficial,  has  been  my  aim  throughout. 

In  neither  volume  have  I  aspired  to  sit  in  the  seat 
of  the  scornful,  but  rather  to  treat  the  questions  touched 
upon  with  a  tolerance,  if  not  a  reverence,  befitting  their 
difficulty  and  weight. 

Holding,  as  I  do,  the  nebular  hypothesis,  I  am  logically 
bound  to  deduce  the  life  of  the  world  from  forces  inherent 
in  the  nebula.  With  this  view,  which  is  set  forth  in  the 
second  volume,  it  seemed  but  fair  to  associate  the  reasons 
which  cause  me  to  conclude  that  every  attempt  made  in 
our  day  to  generate  life  independently  of  antecedent  life 
has  utterly  broken  down. 

A  discourse  on  the  Electric  Light  winds  up  the  second 
volume.  The  incongruity  of  its  position  is  to  be  referred 
to  the  lateness  of  its  delivery. 

(3) 


CONTENTS 


FRAGMENTS    OF    SCIENCE 


VOLUME   ONE 


CHAP.  PAOB 

I.  The  Constitution  of  Nature 7 

II.  Radiation 33 

III.  On   Radiant    Heat   in    Relation    to    the    Color  and 

Chemical  Constitution  of  Bodies        ....      80 

IV.  New  Chemical  Reactions  Produced  by  Light  ,       .       .103 
V.  The  Sky 140 

VI.  Voyage  to  Algeria  to  Observe  the  Eclipse    .       .       .152 

VII.  Niagara »    187 

VIII.  The  Parallel  Roads  of  Glen  Roy 218 

IX.  Alpine  Sctjlpture 243 

X.  Recent  Experiments  on  Fog-Signals  .       ,       .       .268 

XI.  On  the  Study  of  Physics 297 

XII.  On  Crystalline  and  Slaty  Cleavage        .       .       ,       .321 
Xin.  On  Paramagnetic  and  Dt  a  magnetic  Forces      .       .       .    338 

Xrv.  Physical  Basis  of  Solar  Chemistry 847 

XV.  Elementary  Magnetism  .       c       .       .       .       ,       .       .362 

XVI.  On  Force 889 

(5) 


6 


CONTENTS 


CHAP. 


PAOB 

XVII.  Contributions  to  Molecular  Physics     ,       ,       ,       ,407 

XVIII.  Life  and  Letters  of  Faraday 430 

XIX.  The  Copley  Medalist  of  1870    ......    444 

XX.  The  Copley  Medalist  of  1871    ..,,,,    451 

XXI.  Death  by  Lightning 453 

XXII.  Science  and  the  ''Spirits"  .       •       ,       •       •       ,       ,    467 


FRAGMENTS    OF    SCIENCE 


INORGANIC  NATURE 


THE   CONSTITUTION   OF   NATURE* 

WE  cannot  tliink  of  space  as  finite,  for  wherever  in 
imagination  we  erect  a  boundary,  we  are  com- 
pelled to  tliink  of  space  as  existing  beyond  it. 
Thus  by  the  incessant  dissolution  of  limits  we  arrive  at  a 
more  or  less  adequate  idea  of  the  infinity  of  space.  But, 
though  compelled  to  think  of  space  as  unbounded,  there 
is  no  mental  necessity  compelling  us  to  think  of  it  either 
as  filled  or  empty;  whether  it  is  so  or  not  must  be  decided 
by  experiment  and  observation.  That  it  is  not  entirely 
void,  the  starry  heavens  declare;  but  the  question  still  re- 
mains, Are  the  stars  themselves  hung  in  vacuo  ?  Are  the 
vast  regions  which  surround  them,  and  across  which  their 
light  is  propagated,  absolutely  empty  ?  A  century  ago  the 
answer  to  this  question,  founded  on  the  Newtonian  theory, 
would  have  been,  "No,  for  particles  of  light  are  inces- 
santly shot  through  space.'*  The  reply  of  modern  science 
is  also  negative,  but  on  different  grounds.  It  has  the  best 
possible  reasons  for  rejecting  the  idea  of  luminiferous  par- 
ticles; but,  in  support  of  the  conclusion  that  the  celestial 

»  * 'Fortnightly  Review,"  1865,  vol.  ill.,  p.  129. 

(7) 


8  FRAGMENTS    OF   SCIENCE 

spaces  are  occupied  by  matter,  it  is  able  to  offer  proofs 
almost  as  cogent  as  those  which  can  be  adduced  of  the 
existence  of  an  atmosphere  round  the  earth.  Men's  minds, 
indeed,  rose  to  a  conception  of  the  celestial  and  universal 
atmosphere  through  the  study  of  the  terrestrial  and  local 
one.  From  the  phenomena  of  sound,  as  displayed  in  the 
air,  they  ascended  to  the  phenomena  of  light,  as  displayed 
in  the  ether;  which  is  the  name  given  to  the  interstellar 
medium. 

The  notion  of  this  medium  must  not  be  considered  as 
a  vague  or  fanciful  conception  on  the  part  of  scientific 
men.  Of  its  reality  most  of  them  are  as  convinced  as 
they  are  of  the  existence  of  the  sun  and  moon.  The 
luminiferous  ether  has  definite  mechanical  properties.  It 
is  almost  infinitely  more  attenuated  than  any  known  gas, 
but  its  properties  are  those  of  a  solid  rather  than  of  a 
gas.  It  resembles  jelly  rather  than  air.  This  was  not 
the  first  conception  of  the  ether,  but  it  is  that  forced 
upon  us  by  a  more  complete  knowledge  of  its  phenom- 
ena. A  body  thus  constituted  may  have  its  boundaries; 
but,  although  the  ether  may  not  be  co- extensive  with 
space,  it  must  at  all  events  extend  as  far  as  the  most  dis- 
tant visible  stars.  In  fact  it  is  the  vehicle  of  their  light, 
and  without  it  they  could  not  be  seen.  This  all- pervad- 
ing substance  takes  up  their  molecular  tremors,  and  con- 
veys them  with  inconceivable  rapidity  to  our  organs  of 
vision.  It  is  the  transported  shiver  of  bodies  countless 
millions  of  miles  distant,  which  translates  itself  in  human 
consciousness  into  the  splendor  of  the  firmament  at  night. 

If  the  ether  have  a  boundary,  masses  of  ponderable 
matter  might  be  conceived  to  exist  beyond  it,  but  they 
could  emit  no  light.      Beyond  the  ether  dark  suns  might 


THE   CONSTITUTION   OF  NATURE  9 

burn;  there,  Tender  proper  conditions,  combustion  might 
be  carried  on;  fuel  might  consume  unseen,  and  metals  be 
fused  in  invisible  fires.  A  body,  moreover,  once  heated 
there,  would  continue  forever  heated;  a  sun  or  planet 
once  molten,  would  continue  forever  molten.  For,  the 
loss  of  heat  being  simply  the  abstraction  of  molecular 
motion  by  the  ether,  where  this  medium  is  absent  no 
cooling  could  occur.  A  sentient  being,  on  approaching 
a  heated  body  in  this  region,  would  be  conscious  of  no 
augmentation  of  temperature.  The  gradations  of  warmth 
dependent  on  the  laws  of  radiation  would  not  exist,  and 
actual  contact  would  first  reveal  the  heat  of  an  extra 
ethereal  sun. 

Imagine  a  paddle-wheel  placed  in  water  and  caused  to 
rotate.  From  it,  as  a  centre,  waves  would  issue  in  all 
directions,  and  a  wader  as  he  approached  the  place  of  dis- 
turbance would  be  met  by  stronger  and  stronger  waves. 
This  gradual  augmentation  of  the  impression  made  upon 
the  wader  is  exactly  analogous  to  the  augmentation  of 
light  when  we  approach  a  luminous  source.  In  the  one 
case,  however,  the  coarse  common  nerves  of  the  body  suf- 
fice; for  the  other  we  must  have  the  finer  optic  nerve. 
But  suppose  the  water  withdrawn;  the  action  at  a  dis- 
tance would  then  cease,  and,  as  far  as  the  sense  of  touch 
is  concerned,  the  wader  would  be  first  rendered  conscious 
of  the  motion  of  the  wheel  by  the  blow  of  the  paddles. 
The  transference  of  motion  from  the  paddles  to  the  water 
is  mechanically  similar  to  the  transference  of  molecular 
motion  from  the  heated  body  to  the  ether;  and  the  prop- 
agation of  waves  through  the  liquid  is  mechanically  similar 
to  the  propagation  of  light  and  radiant  heat. 

As  far  as  our  knowledge  of  space  extends,  we  are  to 


10  FRAGMENTS   OF  SCIENCE 

conceive  it  as  tlie  holder  of  the  luminiferous  ether,  through 
which  are  interspersed,  at  enormous  distances  apart,  the 
ponderous  nuclei  of  the  stars.  Associated  with  the  star 
that  most  concerns  us  we  have  a  group  of  dark  planetary 
masses  revolving  at  various  distances  round  it,  each  again 
rotating  on  its  own  axis ;  and,  finally,  associated  with  some 
of  these  planets  we  have  dark  bodies  of  minor  note — the 
moons.  Whether  the  other  fixed  stars  have  similar  plan- 
etary companions  or  not  is  to  us  a  matter  of  pure  con- 
jecture, which  may  or  may  not  enter  into  our  conception 
of  the  universe.  But  probably  every  thoughtful  person 
believes,  with  regard  to  those  distant  suns,  that  there  is^ 
in  space,  something  besides  our  system  on  which  they 
shine. 

From  this  general  view  of  the  present  condition  of 
space,  and  of  the  bodies  contained  in  it,  we  pass  to  the 
inquiry  whether  things  were  so  created  at  the  beginning. 
Was  space  furnished  at  once,  by  the  fiat  of  Omnipotence, 
with  these  burning  orbs?  In  presence  of  the  revelations 
of  science  this  view  is  fading  more  and  more.  Behind  the 
orbs  we  now  discern  the  nebulas  from  which  they  have 
been  condensed.  And  without  going  so  far  back  as  the 
nebulse,  the  man  of  science  can  prove  that  out  of  common 
non-luminous  matter  this  whole  pomp  of  stars  might  have 
been  evolved. 

The  law  of  gravitation  enunciated  by  Newton  is,  that 
every  particle  of  matter  in  the  universe  attracts  every 
other  particle  with  a  force  which  diminishes  as  the  square 
of  the  distance  increases.  Thus  the  sun  and  the  earth 
mutually  pull  each  other;  thus  the  earth  and  the  moon 
are  kept  in  company;  the  force  which  holds  every  re- 
spective pair  of  masses  together  being  the  integrated  force 


THE   CONSTITUTION  OF  NATURE  11 

of  their  component  parts.  Under  the  operation  of  this 
force  a  stone  falls  to  the  ground  and  is  warmed  by  the 
shock;  under  its  operation  meteors  plunge  into  our  atmos- 
phere and  rise  to  incandescence.  Showers  of  such  meteors 
doubtless  fall  incessantly  upon  the  sun.  Acted  on  by  this 
force,  the  earth,  were  it  stopped  in  its  orbit  to-morrow, 
would  rush  toward,  and  finally  combine  with,  the  sun. 
Heat  would  also  be  developed  by  this  collision.  Mayer 
first,  and  Helmholtz  and  Thomson  afterward,  have  calcu- 
lated its  amount.  It  would  equal  that  produced  by  the 
combustion  of  more  than  6,000  worlds  of  solid  coal,  all 
this  heat  being  generated  at  the  instant  of  collision.  In 
the  attraction  of  gravity,  therefore,  acting  upon  non-lumi- 
nous matter,  we  have  a  source  of  heat  more  powerful  than 
could  be  derived  from  any  terrestrial  combustion.  And 
were  the  matter  of  the  universe  thrown  in  cold  detached 
fragments  into  space,  and  there  abandoned  to  the  mutual 
gravitation  of  its  own  parts,  the  collision  of  the  fragments 
would  in  the  end  produce  the  fires  of  the  stars. 

The  action  of  gravity  upon  matter  originally  cold  may, 
in  fact,  be  the  origin  of  all  light  and  heat,  and  also  the 
proximate  source  of  such  other  powers  as  are  generated 
by  light  and  heat.  But  we  have  now  to  inquire  what  is 
the  light  and  what  is  the  heat  thus  produced?  This  ques- 
tion has  already  been  answered  in  a  general  way.  Both 
light  and  heat  are  modes  of  motion.  Two  planets  clash 
and  come  to  rest;  their  motion,  considered  as  that  of 
masses,  is  destroyed,  but  it  is  in  great  part  continued  as 
a  motion  of  their  ultimate  particles.  It  is  this  latter  mo- 
tion, taken  up  by  the  ether,  and  propagated  through  it 
with  a  velocity  of  186,000  miles  a  second,  that  comes  to 
us  as  the  light  and  heat  of  suns  and  stars.     The  atoms 


12  FRAGMENTS   OF  SCIENCE 

of  a  hot  body  swing  with  inconceivable  rapidity — billions 
of  times  in  a  second — but  this  power  of  vibration  neces- 
sarily implies  the  operation  of  forces  between  the  atoms 
themselves.  It  reveals  to  us  that  while  they  are  held  to- 
gether by  one  force,  they  are  kept  asunder  by  another, 
their  position  at  any  moment  depending  on  the  equilib- 
rium of  attraction  and  repulsion.  The  atoms  behave  as 
if  connected  by  elastic  springs,  which  oppose  at  the  same 
time  their  approach  and  their  retreat,  but  which  tolerate 
the  vibration  called  heat.  The  molecular  vibration  once 
set  up  is  instantly  shared  with  the  ether,  and  diffused  by 
it  throughout  space. 

We  on  the  earth's  surface  live  night  and  day  in  the 
midst  of  ethereal  commotion.  The  medium  is  never  still. 
The  cloud  canopy  above  us  may  be  thick  enough  to  shut 
out  the  light  of  the  stars ;  but  this  canopy  is  itself  a  warm 
body,  which  radiates  its  thermal  motion  through  the  ether. 
The  earth  also  is  warm,  and  sends  its  heat-pulses  inces- 
santly forth.  It  is  the  waste  of  its  molecular  motion  in 
space  that  chills  the  earth  upon  a  clear  night;  it  is  the 
return  of  thermal  motion  from  the  clouds  which  prevents 
the  earth's  temperature,  on  a  cloudy  night,  from  falling 
so  low.  To  the  conception  of  space  being  filled  we  must 
therefore  add  the  conception  of  its  being  in  a  state  of  in- 
cessant tremor. 

The  sources  of  this  vibration  are  the  ponderable  masses 
of  the  universe.  Let  us  take  a  sample  of  these  and  exam- 
ine it  in  detail.  When  we  look  to  our  planet,  we  find  it 
to  be  an  aggregate  of  solids,  liquids,  and  gases.  Sub- 
jected to  a  sufficiently  low  temperature,  the  two  last  would 
also  assume  the  solid  form.  When  we  look  at  any  one  of 
these  we  generally  find  it  composed  of  still  more  elemen- 


THE   CONSTITUTION  OF  NATURE  13 

tary  parts.  We  learn,  for  example,  that  tlie  water  of  our 
rivers  is  formed  by  the  union,  in  definite  proportions,  of 
two  gases,  oxygen  and  hydrogen.  We  know  how  to  bring 
these  constituents  together,  so  as  to  form  water;  we  also 
know  how  to  analyze  the  water,  and  recover  from  it  its 
two  constituents.  So,  likewise,  as  regards  the  solid  por- 
tions of  the  earth.  Our  chalk  hills,  for  example,  are 
formed  by  a  combination  of  carbon,  oxygen,  and  calcium. 
These  are  the  so-called  elements  the  union  of  which,  in 
definite  proportions,  has  resulted  in  the  formation  of 
chalk.  The  flints  within  the  chalk  we  know  to  be  a  com- 
pound of  oxygen  and  silicium,  called  silica;  and  our  ordi- 
nary clay  is,  for  the  most  part,  formed  by  the  union  of 
silicium,  oxygen,  and  the  well-known  light  metal,  alu- 
minium. By  far  the  greater  portion  of  the  earth's  crust 
is  compounded  of  the  elementary  substances  mentioned  in 
these  few  lines. 

The  principle  of  gravitation  has  been  already  described 
as  an  attraction  which  every  particle  of  matter,  however 
small,  exerts  on  every  other  particle.  With  gravity  there 
is  no  selection;  no  particular  atoms  choose,  by  preference, 
other  particular  atoms  as  objects  of  attraction;  the  attrac- 
tion of  gravitation  is  proportional  simply  to  the  quantity 
of  the  attracting  matter,  regardless  of  its  quality.  But  in 
the  molecular  world  which  we  have  now  entered  matters 
are  otherwise  arranged.  Here  we  have  atoms  between 
which  a  strong  attraction  is  exercised,  and  also  atoms  be- 
tween which  a  weak  attraction  is  exercised.  One  atom 
can  jostle  another  out  of  its  place,  in  virtue  of  a  superior 
force  of  attraction.  But,  though  the  amount  of  force  ex- 
erted varies  thus  from  atom  to  atom,  it  is  still  an  attrac- 
tion of  the  same  mechanical  quality,  if  I  may  use  the  term, 


14  FRAGMENTS   OF  SCIENCE 

as  that  of  gravity  itself.  Its  intensity  might  be  measured 
in  the  same  way,  namely  by  the  amount  of  motion  which 
it  can  generate  in  a  certain  time.  Thus  the  attraction  of 
gravity  at  the  earth's  surface  is  expressed  by  the  number 
82;  because,  when  acting  freely  on  a  body  for  a  second  of 
time,  gravity  imparts  to  the  body  a  velocity  of  thirty-two 
feet  a  second.  In  like  manner  the  mutual  attraction  of 
oxygen  and  hydrogen  might  be  measured  by  the  velocity 
imparted  to  the  atoms  in  their  rushing  together.  Of  course 
such  a  unit  of  time  as  a  second  is  not  here  to  be  thought 
of,  the  whole  interval  required  by  the  atoms  to  cross  the 
minute  spaces  which  separate  them  amounting  only  to  an 
inconceivably  small  fraction  of  a  second. 

It  has  been  stated  that  when  a  body  falls  to  the  earth 
it  is  warmed  by  the  shock.  Here,  to  use  the  terminology 
of  Mayer,  we  have  a  mechanical  combination  of  the  earth 
and  the  body.  Let  us  suffer  the  falling  body  and  the 
earth  to  dwindle  in  imagination  to  the  size  of  atpms,  and 
for  the  attraction  of  gravity  let  us  substitute  that  of  chem- 
ical affinity;  we  have  then  what  is  called  a  chemical  com- 
bination. The  effect  of  the  union  in  this  case  also  is  the 
development  of  heat,  and  from  the  amount  of  heat  gen- 
erated we  can  infer  the  intensity  of  the  atomic  pull.  Meas- 
ured by  ordinary  mechanical  standards,  this  is  enormous. 
Mix  eight  pounds  of  oxygen  with  one  of  hydrogen,  and 
pass  a  spark  through  the  mixture;  the  gases  instantly  com- 
bine, their  atoms  rushing  over  the  Iii^tle  distances  which 
separate  them.  Take  a  weight  of  47,000  pounds  to  an 
elevation  of  1,000  feet  above  the  earth's  surface  and  let  it 
fall;  the  energy  with  which  it  will  strike  the  earth  will  not 
exceed  that  of  the  eight  pounds  of  oxygen  atoms,  as  they 
dash  against  one  pound  of  hydrogen  atoms  to  form  water. 


THE   CONSTITUTION  OF  NATURE  15 

It  is  sometimes  stated  that  gravity  is  distinguished  from 
all  other  forces  by  the  fact  of  its  resisting  conversion  into 
other  forms  of  force.  Chemical  affinity,  it  is  said,  can  be 
converted  into  heat  and  light,  and  these  again  into  mag« 
netism  and  electricity:  but  gravity  refuses  to  be  so  con- 
verted; being  a  force  maintaining  itself  under  all  circum- 
stances, and  not  capable  of  disappearing  to  give  place  to 
another.  The  statement  arises  from  vagueness  of  thought. 
If  by  it  be  meant  that  a  particle  of  matter  can  never  be 
deprived  of  its  weight,  the  assertion  is  correct;  but  the 
law  which  affirms  the  convertibility  of  natural  forces  was 
never  intended,  in  the  minds  of  those  who  understood  it, 
to  affirm  that  such  a  conversion  as  that  here  implied  oc- 
curs in  any  case  whatever.  As  regards  convertibility  into 
heat,  gravity  and  chemical  affinity  stand  on  precisely  the 
same  footing.  The  attraction  in  the  one  case  is  as  inde- 
structible as  in  the  other.  Nobody  affirms  that  when  a 
stone  rests  upon  the  surface  of  the  earth,  the  mutual  at- 
traction of  the  earth  and  stone  is  abolished;  nobody  means 
to  affirm  that  the  mutual  attraction  of  oxygen  for  hydro- 
gen ceases,  after  the  atoms  have  combined  to  form  water. 
What  is  meant,  in  the  case  of  chemical  affinity,  is,  that 
the  pull  of  that  affinity,  acting  through  a  certain  space, 
imparts  a  motion  of  translation  of  the  one  atom  toward 
the  other.  This  motion  is  not  heat,  nor  is  the  force  that 
produces  it  heat.  But  when  the  atoms  strike  and  recoil, 
the  motion  of  translation  is  converted  into  a  motion  of 
vibration,  which  is  heat.  The  vibration,  however,  so  far 
from  causing  the  extinction  of  the  original  attraction,  is 
in  part  carried  on  by  that  attraction.  The  atoms  recoil, 
in  virtue  of  the  elastic  force  which  opposes  actual  contact, 
and  in  the  recoil  they  are  driven  too  far  back.     The  orig- 


16  FRAGMENTS    OF  SCIENCE 

inal  attraction  then  trmmphs  over  the  force  of  recoil,  and 
urges  the  atoms  once  more  together.  Thus,  like  a  pen- 
dulum, the  J  oscillate,  until  their  motion  is  imparted  to  the 
surrounding  ether;  or,  in  other  words,  until  their  heat 
becomes  radiant  heat. 

In  this  sense,  and  in  this  sense  only,  is  chemical  affin- 
ity converted  into  heat.  There  is,  first  of  all,  the  attrac- 
tion between  the  atoms;  there  is,  secondly,  space  between 
them.  Across  this  space  the  attraction  urges  them.  They 
collide,  they  recoil,  they  oscillate.  There  is  here  a  change 
in  the  form  of  the  motion,  but  there  is  no  real  loss.  It  is 
so  with  the  attraction  of  gravity.  To  produce  motion  by 
gravity  space  must  also  intervene  between  the  attracting 
bodies.  When  they  strike  together  motion  is  apparently 
destroyed,  but  in  reality  there  is  no  destruction.  Their 
atoms  are  suddenly  urged  together  by  the  shock;  by  their 
own  perfect  elasticity  these  atoms  recoil;  and  thus  is  set 
up  the  molecular  oscillation  which,  when  communicated 
to  the  proper  nerves,  announces  itself  as  heat. 

It  was  formerly  universally  supposed  that  by  the  col- 
lision of  unelastic  bodies  force  was  destroyed.  Men  saw, 
for  example,  that  when  two  spheres  of  clay,  painter's 
putty,  or  lead,  for  example,  were  urged  together,  the  mo- 
tion possessed  by  the  masses,  prior  to  impact,  was  more 
or  less  annihilated.  They  believed  in  an  absolute  destruc- 
tion of  the  force  of  impact.  Until  recent  times,  indeed, 
no  difficulty  was  experienced  in  believing  this,  whereas, 
at  present,  the  ideas  of  force  and  its  destruction  refuse  to 
be  united  in  most  philosophic  minds.  In  the  collision  of 
elastic  bodies,  on  the  contrary,  it  was  observed  that  the 
motion  with  which  they  clashed  together  was  in  great 
part   restored   by   the   resiliency  of  the  masses,   the  more 


THE   CONSTITUTION  OF  NATURE  17 

perfect  the  elasticity  tlie  more  complete  being  the  restitu- 
tion. This  led  to  the  idea  of  perfectly  elastic  bodies — 
bodies  competent  to  restore  by  their  recoil  the  whole  of 
the  motion  which  they  possessed  before  impact — and  this 
again  to  the  idea  of  the  conservation  of  force,  as  opposed 
to  that  destruction  of  force  which  was  supposed  to  occur 
when  unelastic  bodies  met  in  collision. 

We  now  know  that  the  principle  of  conservation  holds 
equally  good  with  elastic  and  unelastic  bodies.  Perfectly 
elastic  bodies  would  develop  no  heat  on  collision.  They 
would  retain  their  motion  afterward,  though  its  direction 
might  be  changed;  and  it  is  only  when  sensible  motion 
is  wholly  or  partly  destroyed  that  heat  is  generated.  This 
always  occurs  in  unelastic  collision,  the  heat  developed 
being  the  exact  equivalent  of  the  sensible  motion  extin- 
guished. This  heat  virtually  declares  that  the  property 
of  elasticity,  denied  to  the  masses,  exists  among  their 
atoms;  by  the  recoil  and  oscillation  of  which  the  princi- 
ple of  conservation  is  vindicated. 

But  ambiguity  in  the  use  of  the  term  * 'force''  makes 
itself  more  and  more  felt  as  we  proceed.  We  have  called 
the  attraction  of  gravity  a  force,  without  any  reference  to 
motion.  A  body  resting  on  a  shelf  is  as  much  pulled  by 
gravity  as  when,  after  having  been  pushed  off  the  shelf, 
it  falls  toward  the  earth.  We  applied  the  term  force  also 
to  that  molecular  attraction  which  we  called  chemical  affin- 
ity. When,  however,  we  spoke  of  the  conservation  of 
force,  in  the  case  of  elastic  collision,  we  meant  neither  a 
pull  nor  a  push,  which,  as  just  indicated,  might  be  exerted 
upon  inert  matter,  but  we  meant  force  invested  in  motion 
— ^the  vis  viva,  as  it  is  called,  of  the  colliding  masses. 

Force  in  this  form  has  a  definite  mechanical  measure, 


18  FRAGMENTS   OF  SCIENCE 

in  the  amount  of  work  that  it  can  perform.  The  simplest 
form  of  work  is  the  raising  of  a  weight.  A  man  walking 
uphill,  or  upstairs,  with  a  pound  weight  in  his  hand,  to 
an  elevation  say  of  sixteen  feet,  performs  a  certain  amount 
of  work,  over  and  above  the  lifting  of  his  own  body.  If 
he  carries  the  pound  to  a  height  of  thirty-two  feet,  he 
does  twice  the  work;  if  to  a  height  of  forty- eight  feet, 
he  does  three  times  the  work;  if  to  sixty-four  feet,  he 
does  four  times  the  work,  and  so  on.  If,  moreover,  he 
carries  up  two  pounds  instead  of  one,  other  things  being 
equal,  he  does  twice  the  work;  if  three,  four,  or  five 
pounds,  he  does  three,  four,  or  five  times  the  work.  In 
fact,  it  is  plain  that  the  work  performed  depends  on  two 
factors,  the  weight  raised  and  the  height  to  which  it  is 
raised.  It  is  expressed  by  the  product  of  these  two 
factors. 

But  a  body  may  be  caused  to  reach  a  certain  elevation 
in  opposition  to  the  force  of  gravity,  without  being  actu- 
ally carried  up.  If  a  hodman,  for  example,  wished  to 
land  a  brick  at  an  elevation  of  sixteen  feet  above  the 
place  where  he  stood,  he  would  probably  pitch  it  up  to 
the  bricklayer.  He  would  thus  impart,  by  a  sudden  effort, 
a  velocity  to  the  brick  sufficient  to  raise  it  to  the  required 
height;  the  work  accomplished  by  that  effort  being  pre- 
cisely the  same  as  if  he  had  slowly  carried  up  the  brick. 
The  initial  velocity  to  be  imparted,  in  this  case,  is  well 
known.  To  reach  a  height  of  sixteen  feet,  the  brick  must 
quit  the  man's  hand  with  a  velocity  of  thirty-two  feet  a 
second.  It  is  needless  to  say,  that  a  body  starting  with 
any  velocity,  would,  if  wholly  unopposed  or  unaided,  con- 
tinue to  move  forever  with  the  same  velocity.  But  when, 
as  in  the  case  before  us,  the  body  is  thrown  upward,  it 


THE    CONSTITUTION   OF   NATURE  19 

moves  in  opposition  to  gravity,  whicli  incessantly  retards 
its  motion,  and  finally  brings  it  to  rest  at  an  elevation  of 
sixteen  feet.  If  not  here  canght  by  the  bricklayer,  it 
would  return  to  the  hodman  with  an  accelerated  motion, 
and  reach  his  hand  with  the  precise  velocity  it  possessed 
on  quitting  it. 

An  important  relation  between  velocity  and  work  is 
here  to  be  pointed  out.  Supposing  the  hodman  compe- 
tent to  impart  to  the  brick,  at  starting,  a  velocity  of  sixty- 
four  feet  a  second,  or  twice  its  former  velocity,  would  the 
amount  of  work  performed  be  twice  what  it  was  in  the 
first  instance  ?  No ;  it  would  be  four  times  that  quantity ; 
for  a  body  starting  with  twice  the  velocity  of  another  will 
rise  to  four  times  the  height.  In  like  manner,  a  three- 
fold velocity  will  give  a  nine-fold  elevation,  a  four-fold 
velocity  will  give  a  sixteen-fold  elevation,  and  so  on. 
The  height  attained,  then,  is  not  proportional  to  the  ini- 
tial velocity,  but  to  the  square  of  the  velocity.  As  before, 
the  work  is  also  proportional  to  the  weight  elevated. 
Hence  the  work  which  any  moving  mass  whatever  is  com- 
petent to  perform,  in  virtue  of  the  motion  which  it  at  any 
moment  possesses,  is  jointly  proportional  to  its  weight  and 
the  square  of  its  velocity.  Here,  then,  we  have  a  second 
measure  of  work,  in  which  we  simply  translate  the  idea 
of  height  into  its  equivalent  idea  of  motion. 

In  mechanics,  the  product  of  the  mass  of  a  moving 
body  into  the  square  of  its  velocity,  expresses  what  is 
called  the  vis  viva^  or  living  force.  It  is  also  sometimes 
called  the  "mechanical  effect.**  If,  for  example,  a  cannon 
pointed  to  the  zenith  urge  a  ball  upward  with  twice  the 
velocity  imparted  to  a  second  ball,  the  former  will  rise  to 
four  times  the  height  attained  by  the  latter.     If  directed 


20  FRAGMENTS    OF  SCIENCE 

against  a  target,  it  will  also  do  four  times  the  execution. 
Hence  the  importance  of  imparting  a  high  velocity  to  pro- 
jectiles in  war.  Having  thus  cleared  our  way  to  a  per- 
fectly definite  conception  of  the  vis  viva  of  moving  masses, 
we  are  prepared  for  the  announcement  that  the  heat  gen- 
erated by  the  shock  of  a  falling  body  against  the  earth  is 
proportional  to  the  vis  viva  annihilated.  The  heat  is  pro- 
portional to  the  square  of  the  velocity.  In  the  case,  there- 
fore, of  two  cannon-balls  of  equal  weight,  if  one  strike 
a  target  with  twice  the  velocity  of  the  other,  it  will  gen- 
erate four  times  the  heat,  if  with  three  times  the  velocity, 
it  will  generate  nine  times  the  heat,  and  so  on. 

Mr.  Joule  has  shown  that  a  pound  weight  falling  from 
a  height  of  772  feet,  or  772  pounds  falling  through  one 
foot,  will  generate  by  its  collision  with  the  earth  an 
amount  of  heat  sufficient  to  raise  a  pound  of  water  one 
degree  Fahrenheit  in  temperature.  772  ''foot-pounds" 
constitute  the  mechanical  equivalent  of  heat.  Now,  a  body 
falling  from  a  height  of  772  feet,  has,  upon  striking  the 
earth,  a  velocity  of  223  feet  a  second;  and  if  this  velocity 
were  imparted  to  the  body,  by  any  other  means,  the  quan- 
tity of  heat  generated  by  the  stoppage  of  its  motion  would 
be  that  stated  above.  Six  times  that  velocity,  or  1,338 
feet,  would  not  be  an  inordinate  one  for  a  cannon-ball  as 
it  quits  the  gun.  Hence,  a  cannon-ball  moving  with  a 
velocity  of  1,338  feet  a  second,  would,  by  collision,  gen- 
erate an  amount  of  heat  competent  to  raise  its  own  weight 
of  water  36  degrees  Fahrenheit  in  temperature.  If  com- 
posed of  iron,  and  if  all  the  heat  generated  were  concen- 
trated in  the  ball  itself,  its  temperature  would  be  raised 
about  360  degrees  Fahrenheit;  because  one  degree  in  the 
case  of  water  is  equivalent  to   about  ten   degrees   in  the 


THE   CONSTITUTION  OF  NATURE  21 

case  of  iron.  In  artillery  practice,  the  lieat  generated  is 
usually  concentrated  upon  the  front  of  the  bolt,  and  on 
the  portion  of  the  target  first  struck.  By  this  concentra- 
tion the  heat  developed  becomes  sufiiciently  intense  to 
raise  the  dust  of  the  metal  to  incandescence,  a  flash  of 
light  often  accompanying  collision  with  the  target. 

Let  us  now  fix  our  attention  for  a  moment  on  the 
gunpowder  which  urges  the  cannon-ball.  This  is  com- 
posed of  combustible  matter,  which  if  burned  in  the  open 
air  would  yield  a  certain  amount  of  heat.  It  will  not 
yield  this  amount  if  it  perform  the  work  of  urging  a  ball. 
The  heat  then  generated  by  the  gunpowder  will  fall  short 
of  that  produced  in  the  open  air,  by  an  amount  equiva- 
lent to  the  vis  viva  of  the  ball;  and  this  exact  amount  is 
restored  by  the  ball  on  its  collision  with  the  target.  In 
this  perfect  way  are  heat  and  mechanical  motion  con- 
nected. 

Broadly  enunciated,  the  principle  of  the  conservation 
of  force  asserts,  that  the  quantity  of  force  in  the  universe 
is  as  unalterable  as  the  quantity  of  matter;  that  it  is  alike 
impossible  to  create  force  and  to  annihilate  it.  But  in 
what  sense  are  we  to  understand  this  assertion  ?  It  would 
be  manifestly  inapplicable  to  the  force  of  gravity  as  de- 
fined by  Newton;  for  this  is  a  force  varying  inversely  as 
the  square  of  the  distance;  and  to  afiirm  the  constancy  of 
a  varying  force  would  be  self- contradictory.  Yet,  when 
the  question  is  properly  understood,  gravity  forms  no  ex- 
ception to  the  law  of  conservation.  Following  the  method 
pursued  by  Helmholtz,  I  will  here  attempt  an  elementary 
exposition  of  this  law.  Though  destined  in  its  applica- 
tions to  produce  momentous  changes  in  human  thought, 
it  is  not  difficult  of  comprehension. 


22  FRAGMENTS   OF  SCIENCE 

For  the  sake  of  simplicity  we  will  consider  a  particle 
of  matter,  which  we  may  call  r,  to  be  perfectly  fixed,  and 
a  second  movable  particle,  D,  placed  at  a  distance  from  F. 
We  will  assume  that  these  two  particles  attract  each  other 
according  to  the  Newtonian  law.  At  a  certain  distance, 
the  attraction  is  of  a  certain  definite  amount,  which  might 
be  determined  by  means  of  a  spring  balance.  At  half  this 
distance  the  attraction  would  be  augmented  four  times;  at 
a  third  of  the  distance,  nine  times;  at  one-fourth  of  the 
distance,  sixteen  times,  and  so  on.  In  every  case,  the  at- 
traction might  be  measured  by  determining,  with  the 
spring  balance,  the  amount  of  tension  just  sufficient  to 
prevent  d  from  moving  toward  F.  Thus  far  we  have  noth- 
ing whatever  to  do  with  motion;  we  deal  with  statics,  not 
with  dynamics.  We  simply  take  into  account  the  distance 
of  D  from  F,  and  the  pull  exerted  by  gravity  at  that 
distance. 

It  is  customary  in  mechanics  to  represent  the  magni- 
tude of  a  force  by  a  line  of  a  certain  length,  a  force  of 
double  magnitude  being  represented  by  a  line  of  double 
length,  and  so  on.  Placing  then  the  particle  d  at  a  dis- 
tance from  F,  we  can,  in  imagination,  draw  a  straight  line 
from  D  to  F,  and  at  D  erect  a  perpendicular  to  this  line 
which  shall  represent  the  amount  of  the  attraction  exerted 
on  D.  If  D  be  at  a  very  great  distance  from  F,  the  attrac- 
tion will  be  very  small,  and  the  perpendicular  consequently 
very  short.  If  the  distance  be  practically  infinite,  the  at- 
traction is  practically  nil.  Let  us  now  suppose  at  every 
point  in  the  line  joining  F  and  D  a  perpendicular  to  be 
erected,  proportional  in  length  to  the  attraction  exerted  at 
that  point;  we  thus  obtain  an  infinite  number  of  perpen- 
diculars, of  gradually  increasing  length,  as  D  approaches  F. 


THE   CONSTITUTION    OF  NATUBJU  23 

Uniting  the  ends  of  all  these  perpendiculars,  we  obtain  a 
curve,  and  between  this  curve  and  the  straight  line  join- 
ing F  and  D  we  have  an  area  containing  all  the  perpen- 
diculars placed  side  by  side.  Each  one  of  this  infinite 
series  of  perpendiculars  representing  an  attraction,  or  ten- 
sion, as  it  is  sometimes  called,  the  area  just  referred  to 
represents  the  sum  of  the  tensions  exerted  upon  the  par- 
ticle D,  during  its  passage  from  its  first  position  to  F. 

Up  to  the  present  point  we  have  been  dealing  with 
tensions,  not  with  motion.  Thus  far  vis  viva  has  been 
entirely  foreign  to  our  contemplation  of  D  and  F.  Let  us 
now  suppose  D  placed  at  a  practically  infinite  distance 
from  f;  here,  as  stated,  the  pull  of  gravity  would  be  in- 
finitely small,  and  the  perpendicular  representing  it  would 
dwindle  almost  to  a  point.  In  this  position  the  sum  of 
the  tensions  capable  of  being  exerted  on  D  would  be  a 
maximum.  Let  D  now  begin  to  move  in  obedience  to  the 
infinitesimal  attraction  exerted  upon  it.  Motion  being 
once  set  up,  the  idea  of  vis  viva  arises.  In  moving  to- 
ward F  the  particle  D  consumes,  as  it  were,  the  tensions. 
Let  us  fix  our  attention  on  D,  at  any  point  of  the  path 
over  which  it  is  moving.  Between  that  point  and  F  there 
is  a  quantity  of  unused  tensions;  beyond  that  point  the 
tensions  have  been  all  consumed,  but  we  have  in  their 
place  an  equivalent  quantity  of  vis  viva.  After  D  has 
passed  any  point,  the  tension  previously  in  store  at  that 
point  disappears,  but  not  without  having  added,  during 
the  infinitely  small  duration  of  its  action,  a  due  amount 
of  motion  to  that  previously  possessed  by  D.  The  nearer 
D  approaches  to  F,  the  smaller  is  the  sum  of  the  tensions 
remaining,  but  the  greater  is  the  vis  viva ;  the  further  D  is 
from  F,  the  greater  is  the  sum  of  the  unconsumed  ten- 


24  FRAGMENTS   OF  SCIENCE 

sions,  and  the  less  is  tlie  living  force.  Now  the  principle 
of  conservation  affirms,  not  the  constancy  of  the  value  of 
the  tensions  of  gravity,  nor  yet  the  constancy  of  the  vis 
viva,  taken  separately,  but  the  absolute  constancy  of  the 
value  of  both  taken  together.  At  the  beginning  the  vis 
viva  was  zero,  and  the  tension  area  was  a  maximum;  close 
to  F  the  vis  viva  is  a  maximum,  while  the  tension  area  is 
zero.  At  every  other  point,  the  work- producing  power 
of  the  particle  D  consists  in  part  of  vis  viva,  and  in  part 
of  tensions. 

If  gravity,  instead  of  being  attraction,  were  repulsion, 
then,  with  the  particles  in  contact,  the  sum  of  the  tensions 
between  D  and  F  would  be  a  maximum,  and  the  vis  viva 
zero.  If,  in  obedience  to  the  repulsion,  d  moved  away 
from  F,  vis  viva  would  be  generated;  and  the  further  D 
retreated  from  F  the  greater  would  be  its  vis  viva,  and  the 
less  the  amount  of  tension  still  available  for  producing 
motion.  Taking  repulsion  as  well  as  attraction  into  ac- 
count, the  principle  of  the  conservation  of  force  affirms 
that  the  mechanical  value  of  the  tensiojis  and  vires  vivce  of 
the  material  universe,  so  far  as  we  know  it,  is  a  constant 
quantity.  The  universe,  in  short,  possesses  two  kinds  of 
property  which  are  mutually  convertible.  The  diminution 
of  either  carries  with  it  the  enhancement  of  the  other,  the 
total  value  of  the  property  remaining  unchanged. 

The  considerations  here  applied  to  gravity  apply 
equally  to  chemical  affinity.  In  a  mixture  of  oxygen 
and  hydrogen  the  atoms  exist  apart,  but  by  the  applica- 
tion of  proper  means  they  may  be  caused  to  rush  together 
across  the  space  that  separates  them.  While  this  space 
exists,  and  as  long  as  the  atoms  have  not  begun  to  move 
toward  each   other,  we    have   tensions   and  nothing  else. 


THE   CONSTITUTION  OF  NATURE  25 

During  tlieir  motion  toward  eacli  other  the  tensions,  as 
in  the  case  of  gravity,  are  converted  into  vis  viva.  After 
they  clash  we  have  still  vis  viva^  but  in  another  form.  It 
was  translation,  it  is  vibration.  It  was  molecular  transfer, 
it  is  heat. 

It  is  possible  to  reverse  these  processes,  to  unlock  the 
combined  atoms  and  replace  them  in  their  first  positions. 
But,  to  accomplish  this,  as  much  heat  would  be  required 
as  was  generated  by  their  union.  Such  reversals  occur 
daily  and  hourly  in  Nature.  By  the  solar  waves,  the  oxy- 
gen of  water  is  divorced  from  its  hydrogen  in  the  leaves 
of  plants.  As  molecular  vis  viva  the  waves  disappear,  but 
in  so  doing  they  re-endow  the  atoms  of  oxygen  and  hy- 
drogen with  tension.  The  atoms  are  thus  enabled  to  re- 
combine,  and  when  they  do  so  they  restore  the  precise 
amount  of  heat  consumed  in  their  separation.  The  same 
remarks  apply  to  the  compound  of  carbon  and  oxygen, 
called  carbonic  acid,  which  is  exhaled  from  our  lungs, 
produced  by  our  fires,  and  found  sparingly  diffused  every- 
where throughout  the  air.  In  the  leaves  of  plants  the 
sunbeams  also  wrench  the  atoms  of  carbonic  acid  asun- 
der, and  sacrifice  themselves  in  the  act;  but  when  the 
plants  are  burned,  the  amount  of  heat  consumed  in  their 
production  is  restored. 

This,  then,  is  the  rhythmic  play  of  Nature  as  regards 

her  forces.     Throughout  all  her  regions  she  oscillates  from 

tension  to  vis  viva,  from  vis  viva  to  tension.     We  have  the 

same  play  in  the  planetary  system.     The  earth's  orbit  is 

an   ellipse,   one  of  the  foci  of  which  is  occupied   by  the 

sun.     Imagine  the  earth  at  the  most  distant  part  of  the 

orbit.     Her  motion,  and  consequently  her  vis  viva,  is  then 

a  minimum.     The  planet  rounds  the  curve,  and  begins  its 

Science 2 


26  FRAGMENTS   OF   SCIENCE 

approach  to  the  sun.  In  front  it  has  a  store  of  tensions, 
which  are  gradually  consumed,  an  equivalent  amount  of 
vis  viva  being  generated.  When  nearest  to  the  sun  the 
motion,  and  consequently  the  vis  viva^  reach  a  maximum. 
But  here  the  available  tensions  have  been  used  up.  The 
earth  rounds  this  portion  of  the  curve  and  retreats  from 
the  sun.  Tensions  are  now  stored  up,  but  vis  viva  is  lost, 
to  be  again  restored  at  the  expense  of  the  complementary 
force  on  the  opposite  side  of  the  curve.  Thus  beats  the 
heart  of  the  universe,  but  without  increase  or  diminution 
of  its  total  stock  of  force. 

I  have  thus  far  tried  to  steer  clear  amid  confusion,  by 
fixing  the  mind  of  the  reader  upon  things  rather  than 
upon  names.  But  good  names  are  essential;  and  here,  as 
yet,  we  are  not  provided  with  such.  We  have  had  the 
force  of  gravity  and  living  force — two  utterly  distinct 
things.  We  have  had  pulls  and  tensions;  and  we  might 
have  had  the  force  of  heat,  the  force  of  light,  the  force  of 
magnetism,  or  the  force  of  electricity — all  of  which  terms 
have  been  employed  more  or  less  loosely  by  writers  on 
physics.  This  confusion  is  happily  avoided  by  the  intro- 
duction of  the  term  "energy,"  which  embraces  both  ten- 
sion and  vis  viva.  Energy  is  possessed  by  bodies  already" 
in  motion;  it  is  then  actual,  and  we  agree  to  call  \X  actual 
or  dynamic  energy.  It  is  our  old  vis  viva.  On  the  other 
hand,  energy  is  possible  to  bodies  not  in  motion,  but 
which,  in  virtue  of  attraction  or  repulsion,  possess  a  power 
of  motion  which  would  realize  itself  if  all  hindrances  were 
removed.  Looking,  for  example,  at  gravity;  a  body  on 
the  earth's  surface  in  a  position  from  which  it  cannot  fall 
to  a  lower  one  possesses  no  energy.  It  has  neither  mo- 
tion nor  power  of  motion.     But  the  same  body  suspended 


THE   CONSTITUTION    OF   NATURE  27 

at  a  height  above  the  earth  has  a  power  of  motion,  though 
it  may  not  have  exercised  it.  Energy  is  possible  to  such 
a  body,  and  we  agree  to  call  this  potential  energy.  It  con- 
sists of  our  old  tensions.  We,  moreover,  speak  of  the 
conservation  of  energy,  instead  of  the  conservation  of 
force;  and  say  that  the  sum  of  the  potential  and  dynamic 
energies  of  the  material  universe  is  a  constant  quantity. 

A  body  cast  upward  consumes  the  actual  energy  of 
projection,  and  lays  up  potential  energy.  When  it  reaches 
its  utmost  height  all  its  actual  energy  is  consumed,  its  po- 
tential energy  being  then  a  maximum.  When  it  returns, 
there  is  a  reconversion  of  the  potential  into  the  actual.  A 
pendulum  at  the  limit  of  its  swing  possesses  potential  en- 
ergy; at  the  lowest  point  of  its  arc  its  energy  is  all  actual. 
A  patch  of  snow  resting  on  a  mountain  slope  has  poten- 
tial energy;  loosened,  and  shooting  down  as  an  avalanche, 
it  possesses  dynamic  energy.  The  pine-trees  growing  on 
the  Alps  have  potential  energy;  but  rushing  down  the 
Holzrinne  of  the  woodcutters  they  possess  actual  energy. 
The  same  is  true  of  the  mountains  themselves.  As  long 
as  the  rocks  which  compose  them  can  fall  to  a  lower  level, 
they  possess  potential  energy,  which  is  converted  into  ac- 
tual when  the  frost  ruptures  their  cohesion  and  hands 
them  over  to  the  action  of  gravity.  The  stone  avalanches 
of  the  Matterhorn  and  Weisshorn  are  illustrations  in  point. 
The  hammer  of  the  great  bell  of  Westminster,  when  raised 
before  striking,  possesses  potential  energy;  when  it  falls, 
the  energy  becomes  dynamic;  and,  after  the  stroke,  we 
have  the  rhythmic  play  of  potential  and  dynamic  in  the 
vibrations  of  the  bell.  The  same  holds  good  for  the  mo- 
lecular oscillations  of  a  heated  body.  An  atom  is  driven 
against  its  neighbor,  and  recoils.     The  ultimate  amplitude 


28  FRAGMENTS   OF  SCIENCE 

of  the  recoil  being  attained,  the  motion  of  the  atom  in 
that  direction  is  checked,  and  for  an  instant  its  energy 
is  all  potential.  It  is  then  drawn  toward  its  neighbor 
with  accelerated  speed;  thus,  by  attraction,  converting  its 
potential  into  dynamic  energy.  Its  motion  in  this  direc- 
tion is  also  finally  checked,  and  again,  for  an  instant,  its 
energy  is  all  potential.  It  once  more  retreats,  converting, 
by  repulsion,  its  potential  into  dynamic  energy,  till  the 
latter  attains  a  maximum,  after  which  it  is  again  changed 
into  potential  energy.  Thus,  what  is  true  of  the  earth,  as 
she  swings  to  and  fro  in  her  yearly  journey  round  the 
sun,  is  also  true  of  her  minutest  atom.  We  have  wheels 
within  wheels,  and  rhythm  within  rhythm. 

When  a  body  is  heated,  a  change  of  molecular  arrange- 
ment always  occurs,  and  to  produce  this  change  heat  is 
consumed.  Hence,  a  portion  only  of  the  heat  communi- 
cated to  the  body  remains  as  dynamic  energy.  Looking 
back  on  some  of  the  statements  made  at  the  beginning  of 
this  article,  now  that  our  knowledge  is  more  extensive, 
we  see  the  necessity  of  qualifying  them.  When,  for  ex- 
ample, two  bodies  clash,  heat  is  generated;  but  the  heat, 
or  molecular  dynamic  energy,  developed  at  the  moment 
of  collision,  is  not  the  exact  equivalent  of  the  sensible  dy- 
namic energy  destroyed.  The  true  equivalent  is  this  heat, 
plus  the  potential  energy  conferred  upon  the  molecules 
by  the  placing  of  greater  distances  between  them.  This 
molecular  potential  energy  is  afterward,  on  the  cooling 
of  the  body,  converted  into  heat. 

Wherever  two  atoms  capable  of  uniting  together  by 
their  mutual  attractions  exist  separately,  they  form  a  store 
of  potential  energy.  Thus  our  woods,  forests,  and  coal- 
fields on  the  one  hand,   and  our  atmospheric  oxygen  on 


THE   CONSTITUTION   OF   NATURE  29 

the  other,  constitute  a  vast  store  of  energy  of  this  kind — 
vast,  but  far  from  infinite.  We  have,  besides  our  coal- 
fields, metallic  bodies  more  or  less  sparsely  distributed 
through  the  earth's  crust.  These  bodies  can  be  oxi- 
dized; and  hence  they  are,  so  far  as  they  go,  stores  of 
energy.  But  the  attractions  of  the  great  mass  of  the 
earth's  crust  are  already  satisfied,  and  from  them  no 
further  energy  can  possibly  be  obtained.  Ages  ago  the 
elementary  constituents  of  our  rocks  clashed  together  and 
produced  the  motion  of  heat,  which  was  taken  up  by  the 
ether  and  carried  away  through  stellar  space.  It  is  lost 
forever  as  far  as  we  are  concerned.  In  those  ages  the 
hot  conflict  of  carbon,  oxygen,  and  calcium  produced  the 
chalk  and  limestone  hills  which  are  now  cold;  and  from 
this  carbon,  oxygen,  and  calcium  no  further  energy  can 
be  derived.  So  it  is  with  almost  all  the  other  constit- 
uents of  the  earth's  crust.  They  took  their  present  form 
in  obedience  to  molecular  force;  they  turned  their  poten- 
tial energy  into  dynamic,  and  yielded  it  as  radiant  heat 
to  the  universe,  ages  before  man  appeared  upon  this 
planet.  For  him  a  residue  of  potential  energy  remains, 
vast,  truly,  in  relation  to  the  life  and  wants  of  an  individ- 
ual, but  exceedingly  minute  in  comparison  with  the  earth's 
primitive  store. 

To  sum  up.  The  whole  stock  of  energy  or  working- 
power  in  the  world  consists  of  attractions,  repulsions,  and 
motions.  If  the  attractions  and  repulsions  be  so  circum- 
stanced as  to  be  able  to  produce  motion,  they  are  sources 
of  working-power,  but  not  otherwise.  As  stated  a  mo- 
ment ago,  the  attraction  exerted  between  the  earth  and 
a  body  at  a  distance  from  the  earth's  surface,  is  a  source 
of  working-power;    because    the  body   can  be  moved  by 


80  FRAGMENTS   OF  SCIENCE 

the  attraction,  and  in  falling  can  perform  work.  When 
it  rests  at  its  lowest  level  it  is  not  a  source  of  power  or 
energy,  because  it  can  fall  no  further.  But  though  it  has 
ceased  to  be  a  source  of  energy,  the  attraction  of  gravity 
still  acts  as  a  force,  which  holds  the  earth  and  weight 
together. 

The  same  remarks  apply  to  attracting  atoms  and  mole- 
cules. As  long  as  distance  separates  them,  they  can  move 
across  it  in  obedience  to  the  attraction;  and  the  motion 
thus  produced  may,  by  proper  appliances,  be  caused  to 
perform  mechanical  work.  When,  for  example,  two  atoms 
of  hydrogen  unite  with  one  of  oxygen,  to  form  water,  the 
atoms  are  first  drawn  toward  each  other — they  move,  they 
clash,  and  then,  by  virtue  of  their  resiliency,  they  recoil 
and  quiver.  To  this  quivering  motion  we  give  the  name 
of  heat.  This  atomic  vibration  is  merely  the  redistribu- 
tion of  the  motion  produced  by  the  chemical  affinity;  and 
this  is  the  only  sense  in  which  chemical  affinity  can  be 
said  to  be  converted  into  heat.  We  must  not  imagine  the 
chemical  attraction  destroyed,  or  converted  into  anything 
else.  For  the  atoms,  when  mutually  clasped  to  form  a 
molecule  of  water,  are  held  together  by  the  very  attrac- 
tion which  first  drew  them  toward  each  other.  That 
which  has  really  been  expended  is  the  pull  exerted 
through  the  space  by  which  the  distance  between  the 
atoms  has  been  diminished. 

If  this  be  understood,  it  will  be  at  once  seen  that 
gravity,  as  before  insisted  on,  may,  in  this  sense,  be  said 
to  be  convertible  into  heat;  that  it  is  in  reality  no  more 
an  outstanding  and  inconvertible  agent,  as  it  is  sometimes 
stated  to  be,  than  is  chemical  affinity.  By  the  exertion  of 
a  certain  pull  through  a  certain  space,  a  body  is  caused 


THE   CONSTITUTION    OF  NATURE  81 

to  clash  with  a  certain  definite  velocity  against  the  earth. 
Heat  is  thereby  developed,  and  this  is  the  only  sense  in 
which  gravity  can  be  said  to  be  converted  into  heat.  In 
no  case  is  the  force  which  produces  the  motion  annihilated 
or  changed  into  anything  else.  The  mutual  attraction  of 
the  earth  and  weight  exists  when  they  are  in  contact,  as 
when  they  were  separate;  but  the  ability  of  that  attrac- 
tion to  employ  itself  in  the  production  of  motion  does  not 
exist. 

The  transformation,  in  this  case,  is  easily  followed  by 
the  mind's  eye.  First,  the  weight  as  a  whole  is  set  in 
motion  by  the  attraction  of  gravity.  This  motion  of  the 
mass  is  arrested  by  collision  with  the  earth,  being  broken 
up  into  molecular  tremors,  to  which  we  give  the  name  of 
heat. 

And  when  we  reverse  the  process,  and  employ  those 
tremors  of  heat  to  raise  a  weight — which  is  done  through 
the  intermediation  of  an  elastic  fluid  in  the  steam-engine 
— a  certain  definite  portion  of  the  molecular  motion  is  con- 
sumed. In  this  sense,  and  in  this  sense  only,  can  the  heat 
be  said  to  be  converted  into  gravity;  or,  more  cprrectly, 
into  potential  energy  of  gravity.  Here  the  destruction  of 
the  heat  has  created  no  new  attraction;  but  the  old  attrac- 
tion has  conferred  upon  it  a  power  of  exerting  a  certain 
definite  pull,  between  the  starting-point  of  the  falling 
weight  and  the   earth. 

When,  therefore,  writers  on  the  conservation  of  energy 
speak  of  tensions  being  ''consumed"  and  "generated," 
they  do  not  mean  thereby  that  old  attractions  have  been 
annihilated,  and  new  ones  brought  into  existence,  but  that, 
in  the  one  case,  the  power  of  the  attraction  to  produce 
motion  has  been  diminished  by  the  shortening  of  the  dis- 


32  FRAGMENTS   OF  SCIENCE 

tance  between  the  attracting  bodies,  wbile,  in  the  other 
case,  the  power  of  producing  motion  has  been  augmented 
bj  the  increase  of  the  distance.  These  remarks  apply  to 
all  bodies,  whether  they  be  sensible  masses  or  molecules. 
Of  the  inner  quality  that  enables  matter  to  attract  mat- 
ter we  know  nothing;  and  the  law  of  conservation  makes 
no  statement  regarding  that  quality.  It  takes  the  facts  of 
attraction  as  they  stand,  and  affirms  only  the  constancy 
of  working- power.  That  power  may  exist  in  the  form  of 
motion;  or  it  may  exist  in  the  form  of  force,  ivith  dis- 
tance to  act  through.  The  former  is  dynamic  energy,  the 
latter  is  potential  energy,  the  constancy  of  the  sum  of  both 
being  affirmed  by  the  law  of  conservation.  The  converti- 
bility of  natural  forces  consists  solely  in  transformations 
of  dynamic  into  potential,  and  of  potential  into  dynamic, 
energy.  In  no  other  sense  has  the  convertibility  of  force 
any  scientific  meaning. 


Grave  errors  have  been  entertained  as  to  what  is  really 
intended  to  be  conserved  by  the  doctrine  of  conservation. 
This  exposition  I  hope  will  tend  to  remove  them. 


II 

RADIATION' 

1.    Visible  and  Invisible  Badiation 

BETWEEN  the  mind  of  man  and  the  outer  world  are 
interposed  the  nerves  of   the   human   body,   which 
translate,  or  enable  the  mind  to  translate,  the  im- 
pressions  of  that  world   into   facts   of    consciousness   and 
thought. 

Different  nerves  are  suited  to  the  perception  of  differ- 
ent impressions.  We  do  not  see  with  the  ear,  nor  hear 
with  the  eye,  nor  are  we  rendered  sensible  of  sound  by 
the  nerves  of  the  tongue.  Out  of  the  general  assemblage 
of  physical  actions,  each  nerve,  or  group  of  nerves,  selects 
and  responds  to  those  for  the  perception  of  which  it  is 
specially  organized. 

The  optic  nerve  passes  from  the  brain  to  the  back  of 
the  eyeball  and  there  spreads  out,  to  form  the  retina,  a 
web  of  nerve  filaments,  on  which  the  images  of  external 
objects  are  projected  by  the  optical  portion  of  the  eye. 
This  nerve  is  limited  to  the  apprehension  of  the  phenom- 
ena of  radiation,  and,  notwithstanding  its  marvellous  sen- 
sibility to  certain  impressions  of  this  class,  it  is  singularly 
obtuse  to  other  impressions. 

'  The  Rede  Lecture  delivered  in  the  Senate  House  before  the  University  of 
Cambridge,  May  16,  1865. 

(3S) 


84  FRAGMENTS   OF  SCIENCE 

Nor  does  tlie  optic  nerve  embrace  the  entire  range  even 
of  radiation.  Some  rays,  when  they  reach  it,  are  incom- 
petent to  evoke  its  power,  while  others  never  reach  it  at 
all,  being  absorbed  by  the  humors  of  the  eye.  To  all  rays 
which,  whether  they  reach  the  retina  or  not,  fail  to  excite 
vision,  we  give  the  name  of  invisible  or  obscure  rays.  All 
non- luminous  bodies  emit  such  rays.  There  is  no  body 
in  nature  absolutely  cold,  and  every  body  not  absolutely 
cold  emits  rays  of  heat.  But  to  render  radiant  heat  fit  to 
affect  the  optic  nerve  a  certain  temperature  is  necessary. 
A  cool  poker  thrust  into  a  fire  remains  dark  for  a  time, 
but  when  its  temperature  has  become  equal  to  that  of  the 
surrounding  coals,  it  glows  like  them.  In  like  manner,  if 
a  current  of  electricity,  of  gradually  increasing  strength, 
be  sent  through  a  wire  of  the  refractory  metal  platinum, 
the  wire  first  becomes  sensibly  warm  to  the  touch;  for  a 
time  its  heat  augments,  still,  however,  remaining  obscure; 
at  length  we  can  no  longer  touch  the  metal  with  impu- 
nity; and  at  a  certain  definite  temperature  it  emits  a  fee- 
ble red  light.  As  the  current  augments  in  power  the 
light  augments  in  brilliancy,  until  finally  the  wire  ap- 
pears of  a  dazzling  white.  The  light  which  it  now  emits 
is  similar  to  that  of  the  sun. 

By  means  of  a  prism  Sir  Isaac  Newton  unravelled  the 
texture  of  solar  light,  and  by  the  same  simple  instrument 
we  can  investigate  the  luminous  changes  of  our  platinum 
wire.  In  passing  through  the  prism  all  its  rays  (and  they 
are  infinite  in  variety)  are  bent  or  refracted  from  their 
straight  course;  and,  as  different  rays  are  differently  re- 
fracted by  the  prism,  we  are  by  it  enabled  to  separate  one 
class  of  rays  from  another.  By  such  prismatic  analysis 
Dr.  Draper  has  shown,  that  when  the  platinum  wire  first 


RADIATION  85 

begins  to  glow  the  light  emitted  is  sensibly  red.  As  tbe 
glow  augments  tbe  red  becomes  more  brilliant,  but  at  the 
same  time  orange  rays  are  added  to  the  emission.  Aug- 
menting the  temperature  still  further,  yellow  rays  appear 
beside  the  orange;  after  the  yellow,  green  rays  are  emitted; 
and  after  the  green  come,  in  succession,  blue,  indigo,  and 
violet  rays.  To  display  all  these  colors  at  the  same  time 
the  platin^um  wire  must  be  white-hot:  the  impression  of 
whiteness  being  in  fact  produced  by  the  simultaneous 
action  of  all  these  colors  on  the  optic  nerve. 

In  the  experiment  just  described  we  began  with  a  pla- 
tinum wire  at  an  ordinary  temperature,  and  gradually 
raised  it  to  a  white  heat.  At  the  beginning,  and  even 
before  the  electric  current  had  acted  at  all  upon  the  wire, 
it  emitted  invisible  rays.  For  some  time  after  the  action 
of  the  current  had  commenced,  and  even  for  a  time  after 
the  wire  had  become  intolerable  to  the  touch,  its  radiation 
was  still  invisible.  The  question  now  arises,  What  be- 
comes of  these  invisible  rays  when  the  visible  ones  make 
their  appearance?  It  will  be  proved  in  the  sequel  that 
they  maintain  themselves  in  the  radiation ;  that  a  ray  once 
emitted  continues  to  be  emitted  when  the  temperature  is 
increased,  and  hence  the  emission  from  our  platinum  wire, 
even  when  it  has  attained  its  maximum  brilliancy,  consists ' 
of  a  mixture  of  visible  and  invisible  rays.  If,  instead  of 
the  platinum  wire,  the  earth  itself  were  raised  to  incan- 
descence, the  obscure  radiation  which  it  now  emits  would 
continue  to  be  emitted.  To  reach  incandescence  the  planet 
would  have  to  pass  through  all  the  stages  of  non-lumin  ^ZB 
radiation,  and  the  final  emission  would  embrace  the  rays 
of  all  these  stages.  There  can  hardly  be  a  doubt  that, 
from  the  sun  itself,  rays  proceed  similar  in  kind  to  those 


86  FRAGMENTS   OF  SCIENCE 

which  the  dark  earth  pours  nightly  into  space.  In  fact, 
the  various  kinds  of  obscure  rays  emitted  by  all  the  plan- 
ets of  our  system  are  included  in  the  present  radiation  of 
the  sun. 

The  great  pioneer  in  this  domain  of  science  was  Sir 
William  Herschel.  Causing  a  beam  of  solar  light  to  pass 
through  a  prism,  he  resolved  it  into  its  colored  constitu- 
ents; he  formed  what  is  technically  called  the  solar  spec- 
trum. Exposing  thermometers  to  the  successive  colors,  he 
determined  their  heating  power,  and  found  it  to  augment 
from  the  violet  or  most  refracted  end,  to  the  red  or  least 
refracted  end  of  the  spectrum.  But  he  did  not  stop  here. 
Pushing  his  thermometers  into  the  dark  space  beyond  the 
red  he  found  that,  though  the  light  had  disappeared,  the 
radiant  heat  falling  on  the  instruments  was  more  intense 
than  that  at  any  visible  part  of  the  spectrum.  In  fact,  Sir 
William  Herschel  showed,  and  his  results  have  been  veri- 
fied by  various  philosophers  since  his  time,  that,  besides 
its  luminous  rays,  the  sun  pours  forth  a  multitude  of  other 
rays,  more  powerfully  calorific  than  the  luminous  ones,  but 
entirely  unsuited  to  the  purposes  of  vision. 

At  the  less  refrangible  end  of  the  solar  spectrum,  then, 
the  range  of  the  sun's  radiation  is  not  limited  by  that  of 
the  eye.  The  same  statement  applies  to  the  more  refran- 
gible end.  Bitter  discovered  the  extension  of  the  spectrum 
into  the  invisible  region  beyond  the  violet;  and,  in  recent 
times,  this  ultra-violet  emission  has  had  peculiar  interest 
conferred  upon  it  by  the  admirable  researches  of  Professor 
Stokes.  The  complete  spectrum  of  the  sun  consists,  there- 
fore, of  three  distinct  parts:  first,  of  ultra-red  rays  of  high 
heating  power,  but  unsuited  to  the  purposes  of  vision; 
secondly,   of  luminous  rays  which  display  the  succession 


RADIATION  87 

of  colors,  red,  orange,  yellow,  green,  blue,  indigo,  violet; 
thirdly,  of  ultra-violet  rays  whicli,  like  the  ultra- red  ones, 
are  incompetent  to  excite  vision,  but  which,  unlike  the 
ultra- red  rays,  possess  a  very  feeble  heating  power.  In 
consequence,  however,  of  their  chemical  energy  these  ultra- 
violet rays  are  of  the  utmost  importance  to  the  organic 
world. 

2.    Origin  and  Character  of  Radiation.     The  Ether 

"When  we  see  a  platinum  wire  raised  gradually  to  a 
white  heat,  and  emitting  in  succession  all  the  colors  of  the 
spectrum,  we  are  simply  conscious  of  a  series  of  changes  in. 
the  condition  of  our  own  eyes.  "We  do  not  see  the  actions 
in  which  these  successive  colors  originate,  but  the  mind 
irresistibly  infers  that  the  appearance  of  the  colors  corre- 
sponds to  certain  contemporaneous  changes  in  the  wire. 
"What  is  the  nature  of  these  changes?  In  virtue  of  what 
condition  does  the  wire  radiate  at  all  ?  We  must  now 
look  from  the  wire,  as  a  whole,  to  its  constituent  atoms. 
Could  we  see  those  atoms,  even  before  the  electric  current 
has  begun  to  act  upon  them,  we  should  find  them  in  a 
state  of  vibration.  In  this  vibration,  indeed,  consists  such 
warmth  as  the  wire  then  possesses.  Locke  enunciated  this 
idea  with  great  precision,  and  it  has  been  placed  beyond 
the  pale  of  doubt  by  the  excellent  quantitative  researches 
of  Mr.  Joule.  "Heat,"  says  Locke,  "is  a  very  brisk  agi- 
tation of  the  insensible  parts  of  the  object,  which  produce 
in  us  that  sensation  from  which  we  denominate  the  object 
hot:  so  what  in  our  sensations  is  heat  in  the  object  is  noth- 
ing but  motion,''''  When  the  electric  current,  still  feeble, 
begins  to  pass  through  the  wire,  its  first  act  is  to  intensify 
the  vibrations  already  existing,   by  causing  the  atoms  to 


88  FRAGMENTS   OF  SCIENCE 

swing  through  wider  ranges.  Technically  speaking,  the 
amplitudes  of  the  oscillations  are  increased.  The  current 
does  this,  however,  without  altering  the  periods  of  the  old 
vibrations,  or  the  times  in  which  they  were  executed.  But 
besides  intensifying  the  old  vibrations  the  current  gener- 
ates new  and  more  rapid  ones,  and  when  a  certain  definite 
rapidity  has  been  attained,  the  wire  begins  to  glow.  The 
color  first  exhibited  is  red,  which  corresponds  to  the  low- 
est rate  of  vibration  of  which  the  eye  is  able  to  take  cog- 
nizance. By  augmenting  the  strength  of  the  electric  cur- 
rent more  rapid  vibrations  are  introduced,  and  orange  rays 
appear.  A  quicker  rate  of  vibration  produces  yellow,  a 
still  quicker  green;  and  by  further  augmenting  the  rapid- 
ity, we  pass  through  blue,  indigo,  and  violet,  to  the  ex- 
treme ultra-violet  rays. 

Such  are  the  changes  recognized  by  the  mind  in  the 
wire  itself,  as  concurrent  with  the  visual  changes  taking 
place  in  the  eye.  But  what  connects  the  wire  with  this 
organ  ?  By  what  means  does  it  send  such  intelligence  of 
its  varying  condition  to  the  optic  nerve?  Heat  being,  as 
defined  by  Locke,  **a  very  brisk  agitation  of  the  insen- 
sible parts  of  an  object,**  it  is  readily  conceivable  that 
on  touching  a  heated  body  the  agitation  may  communicate 
itself  to  the  adjacent  nerves,  and  announce  itself  to  them 
as  light  or  heat.  But  the  optic  nerve  does  not  touch  the 
hot  platinum,  and  hence  the  pertinence  of  the  question, 
By  what  agency  are  the  vibrations  of  the  wire  transmitted 
to  the  eye  ? 

The  answer  to  this  question  involves  one  of  the  most 
important  physical  conceptions  that  the  mind  of  man  has 
yet  achieved :  the  conception  of  a  medium  filling  space  and 
fitted  mechanically  for  the  transmission  of  the  vibrations 


RADIATION  89 

of  light  and  heat,  as  air  is  fitted  for  the  transmission  of 
sound.  This  medium  is  called  the  lumini/erous  ether. 
Every  vibration  of  every  atom  of  our  platinum  wire  raises 
in  this  ether  a  wave,  which  speeds  through  it  at  the  rate 
of  186,000  miles  a  second.  The  ether  suffers  no  rupture 
of  continuity  at  the  surface  of  the  eye,  the  inter-molecular 
spaces  of  the  various  humors  are  filled  with  it;  hence  the 
waves  generated  by  the  glowing  platinum  can  cross  these 
humors  and  impinge  on  the  optic  nerve  at  the  back  of  the 
eye. '  Thus  the  sensation  of  light  reduces  itself  to  the  ac- 
ceptance of  motion.  Up  to  this  point  we  deal  with  pure 
mechanics;  but  the  subsequent  translation  of  the  shock  of 
the  ethereal  waves  into  consciousness  eludes  mechanical 
science.  As  an  oar  dipping  into  the  Cam  generates  sys- 
tems of  waves,  which,  speeding  from  the  centre  of  dis- 
turbance, finally  stir  the  sedges  on  the  river's  bank,  so 
do  the  vibrating  atoms  generate  in  the  surrounding  ether 
undulations,  which  finally  stir  the  filaments  of  the  retina. 
The  motion  thus  imparted  is  transmitted  with  measurable, 
and  not  very  great,  velocity  to  the  brain,  where,  by  a  proc- 
ess which  the  science  of  mechanics  does  not  even  tend  to 
unravel,  the  tremor  of  the  nervous  matter  is  converted  into 
the  conscious  impression  of  light. 

Darkness  might  then  be  defined  as  ether  at  rest;  light 
as  ether  in  motion.  But  in  reality  the  ether  is  never  at 
rest,  for  in  the  absence  of  light-waves  we  have  heat-waves 
always  speeding  through  it.  In  the  spaces  of  the  universe 
both  classes  of  undulations  incessantly  commingle.  Here 
the  waves  issuing  from  uncounted  centres  cross,  coincide, 
oppose,  and  pass  through  each  other,  without  confusion  or 

*  The  action  here  described  is  analogous  to  the  passage  of  sound-waves 
through  thick  felt  whose  interstices  are  occupied  by  air. 


40  FRAGMENTS   OF  SCIENCE 

ultimate  extinction.  Every  star  is  seen  across  the  entan- 
glement of  wave-motions  produced  bj  all  other  stars.  It 
is  the  ceaseless  thrill  caused  by  those  distant  orbs  collec- 
tively in  the  ether  that  constitutes  what  we  call  the  "tem- 
perature of  space."  As  the  air  of  a  room  accommodates 
itself  to  the  requirements  of  an  orchestra,  transmitting  each 
vibration  of  every  pipe  and  string,  so  does  the  interstellar 
ether  accommodate  itself  to  the  requirements  of  light  and 
heat.  Its  waves  mingle  in  space  without  disorder,  each 
being  endowed  with  an  individuality  as  indestructible  as 
if  it  alone  had  disturbed  the  universal  repose. 

AIL  vagueness  with  regard  to  the  use  of  the  terms 
"radiation"  and  "absorption"  will  now  disappear.  Eadi- 
ation  is  the  communication  of  vibratory  motion  to  the 
ether;  and  when  a  body  is  said  to  be  chilled  by  radia- 
tion, as  for  example  the  grass  of  a  meadow  on  a  starlight 
night,  the  meaning  is,  that  the  molecules  of  the  grass  have 
lost  a  portion  of  their  motion,  by  imparting  it  to  the  me- 
dium in  which  they  vibrate.  On  the  other  hand,  the  waves 
of  ether  may  so  strike  against  the  molecules  of  a  body  ex- 
posed to  their  action  as  to  yield  up  their  motion  to  the 
latter;  and  in  this  transfer  of  the  motion  from  the  ether 
to  the  molecules  consists  the  absorption  of  radiant  heat. 
All  the  phenomena  of  heat  are  in  this  way  reducible  to 
interchanges  of  motion;  and  it  is  purely  as  the  recipients 
or  the  donors  of  this  motion  that  we  ourselves  become 
conscious  of  the  action  of  heat  and  cold. 

3.  The  Atomic  Theory  in  reference  to  the  Ether 

The  word  "atoms"  has  been  more  than  once  employed 
in  this  discourse.  Chemists  have  taught  us  that  all  matter 
is  reducible  to  certain  elementary  forms  to  which  they  give 


RADIATION'  41 

this  name.  These  atoms  are  endowed  with  powers  of  mu- 
tual attraction,  and  under  suitable  circumstances  they  coa- 
lesce to  form  compounds.  Thus  oxygen  and  hydrogen  are 
elements  when  separate,  or  merely  mixed,  but  they  may  be 
made  to  comhine  so  as  to  form  molecules,  each  consisting 
of  two  atoms  of  hydrogen  and  one  of  oxygen.  In  this  con- 
dition they  constitute  water.  So  also  chlorine  and  sodium 
are  elements,  the  former  a  pungent  gas,  the  latter  a  soft 
metal;  and  they  unite  together  to  form  chloride  of  sodium 
or  common  salt.  In  the  same  way  the  element  nitrogen 
combines  with  hydrogen,  in  the  proportion  of  one  atom  of 
the  former  to  three  of  the  latter,  to  form  ammonia.  Pict- 
uring in  imagination  the  atoms  of  elementary  bodies  as 
little  spheres,  the  molecules  of  compound  bodies  must  be 
pictured  as  groups  of  such  spheres.  This  is  the  atomic 
theory  as  Dalton  conceived  it.  Now,  if  this  theory  have 
any  foundation  in  fact,  and  if  the  theory  of  an  ether  per- 
vading space,  and  constituting  the  vehicle  of  atomic  mo- 
tion, be  founded  in  fact,  it  is  surely  of  interest  to  examine 
whether  the  vibrations  of  elementary  bodies  are  modified 
by  the  act  of  combination — whether  as  regards  radiation 
and  absorption,  or,  in  other  words,  whether  as  regards  the 
communication  of  motion  to  the  ether,  and  the  acceptance 
of  motion  from  it,  the  deportment  of  the  uncombined  atoma 
will  be  different  from  that  of  the  combined. 

4.  Absorption  of  Radiant  Heat  by  Gases 

We  have  now  to  submit  these  considerations  to  the 
only  test  by  which  they  can  be  tried,  namely,  that  of 
experiment.  An  experiment  is  well  defined  as  a  question, 
put  to  Nature;  but,  to  avoid  the  risk  of  asking  amiss,  we 
ought  to  purify  the  question  from  all  adjuncts  which  do 


42  FRAGMENTS    OF  SCIENCE 

not  necessarily  belong  to  it.  Matter  has  been  sbown  to  be 
composed  of  elementary  constituents,  by  tbe  compounding 
of  wbich  all  its  varieties  are  produced.  But,  besides  the 
chemical  unions  which  they  form,  both  elementary  and 
compound  bodies  can  unite  in  another  and  less  intimate 
way.  Grases  and  vapors  aggregate  to  liquids  and  solids, 
without  any  change  of  their  chemical  nature.  We  do  not 
yet  know  how  the  transmission  of  radiant  heat  may  be 
affected  by  the  entanglement  due  to  cohesion;  and,  as  our 
object  now  is  to  examine  the  influence  of  chemical  union 
alone,  we  shall  render  our  experiments  more  pure  by  lib- 
erating the  atoms  and  molecules  entirely  from  the  bonds 
of  cohesion,  and  employing  them  in  the  gaseous  or  vapor- 
ous form. 

Let  us  endeavor  to  obtain  a  perfectly  clear  mental  im- 
age of  the  problem  now  before  us.  Limiting  in  the  first 
place  our  inquiries  to  the  phenomena  of  absorption,  we 
have  to  picture  a  succession  of  waves  issuing  from  a  radi- 
ant source  and  passing  through  a  gas;  some  of  them  strik- 
ing against  the  gaseous  molecules  and  yielding  up  their 
motion  to  the  latter;  others  gliding  round  the  molecules, 
or  passing  through  the  inter- molecular  spaces  without  ap- 
parent hindrance.  The  problem  before  us  is  to  determine 
whether  such  free  molecules  have  any  power  whatever  to 
stop  the  waves  of  heat;  and  if  so,  whether  different  mole- 
cules possess  this  power  in  different  degrees. 

In  examining  the  problem  let  us  fall  back  upon  an 
actual  piece  of  work,  choosing  as  the  source  of  our  heat- 
waves a  plate  of  copper,  against  the  back  of  which  a 
steady  sheet  of  flame  is  permitted  to  play.  On  emerg- 
ing from  the  copper,  the  waves,  in  the  first  instance,  pass 
through  a  space  devoid  of  air,   and  then  enter   a   hollow 


RADIATION'  43 

glass  cylinder,  three  feet  long  and  three  inches  wide.  The 
two  ends  of  this  cylinder  are  stopped  by  two  plates  of 
rock-salt,  a  solid  substance  which  offers  a  scarcely  sensi- 
ble obstacle  to  the  passage  of  the  calorific  waves.  After 
passing  through  the  tube,  the  radiant  heat  falls  upon  the 
anterior  face  of  a  thermo-electric  pile,*  wHich  instantly 
converts  the  heat  into  an  electric  current.  This  current 
conducted  round  a  magnetic  needle  deflects  it,  and  the 
magnitude  of  the  deflection  is  a  measure  of  the  heat  fall- 
ing upon  the  pile.  This  famous  instrument,  and  not  an 
ordinary  thermometer,  is  what  we  shall  use  in  these  in- 
quiries, but  we  shall  use  it  in  a  somewhat  novel  way.  As 
long  as  the  two  opposite  faces  of  the  thermo-electric  pile 
are  kept  at  the  same  temperature,  no  matter  how  high  that 
may  be,  there  is  no  current  generated.  The  current  is  a 
consequence  of  a  difference  of  temperature  between  the 
two  opposite  faces  of  the  pile.  Hence,  if  after  the  ante- 
rior face  has  received  the  heat  from  our  radiating  source, 
a  second  source,  which  we  may  call  the  compensating 
source,  be  permitted  to  radiate  against  the  posterior  face, 
this  latter  radiation  will  tend  to  neutralize  the  former. 
When  the  neutralization  is  perfect,  the  magnetic  needle 
connected  with  the  pile  is  no  longer  deflected,  but  points 
to  the  zero  of  the  graduated  circle  over  which  it  hangs. 

And  now  let  us  suppose  the  glass  tube,  through  which 
the  waves  from  the  heated  plate  of  copper  are  passing,  to 
be  exhausted  by  an  air-pump,  the  two  sources  of  heat  act- 
ing at  the  same  time  on  the  two  opposite  faces  of  the  pile. 
When,  by  means  of  an  adjusting  screen,   perfectly  equal 


*  In  the  Appendix  to  the  first  chapter  of  *'Heat  as  a  Mode  of  Motion,"  the 
construction  of  the  thermo-electric  pile  is  fully  explained. 


44  FRAGMENTS    OF  SCIENCE 

quantities  of  heat  are  imparted  to  the  two  faces,  the  needle 
points  to  zero.  Let  any  gas  be  now  permitted  to  enter  the 
exhausted  tube;  if  its  molecules  possess  any  power  of  in- 
tercepting the  calorific  waves,  the  equilibrium  previously 
existing  will  be  destroyed,  the  compensating  source  will 
triumph,  and  a  deflection  of  the  magnetic  needle  will  be 
the  immediate  consequence.  From  the  deflections  thus 
produced  by  different  gases  we  can  readily  deduce  the 
relative  amounts  of  wave- motion  which  their  molecules 
intercept. 

In  this  way  the  substances  mentioned  in  the  following 
table  were  examined,  a  small  portion  only  of  each  being 
admitted  into  the  glass  tube.  The  quantity  admitted  in 
each  case  was  just  sufficient  to  depress  a  column  of  mer- 
cury associated  with  the  tube  one  inch:  in  other  words, 
the  gases  were  examined  at  a  pressure  of  one- thirtieth  of 
an  atmosphere.  The  numbers  in  the  table  express  the 
relative  amounts  of  wave- motion  absorbed  by  the  respec- 
tive gases,  the  quantity  intercepted  by  atmospheric  air 
being  taken  as  unity. 

Radiation  through  Gases 

Relative 
Name  of  gas  absorption 

Air 1 

Oxygen     1 

.     Nitrogen 1 

Hydrogen 1 

Carbonic  oxide  ........  *750 

Carbonic  acid   .         . 912 

Hydrochloric  acid 1,005 

Nitric  oxide 1,590 

Nitrous  oxide 1,860 

Sulphide  of  hydrogen 2,100 

Ammonia           .       *                             ....  5,460 

defiant  gas       .         .                            ....  6,030 

Sulphurous  acid        ......  6,480 


RADIATION  45 

Every  gas  in  this  table  is  perfectly  transparent  to  light, 
that  is  to  say,  all  waves  within  the  limits  of  the  visible 
spectrum  pass  through  it  without  obstruction;  but  for  the 
waves  of  slower  period,  emanating  from  our  heated  plate 
of  copper,  enormous  differences  of  absorptive  power  are 
manifested.  These  differences  illustrate  in  the  most  un- 
expected manner  the  influence  of  chemical  combination. 
Thus  the  elementary  gases,  oxygen,  hydrogen,  and  nitro- 
gen, and  the  mixture  atmospheric  air,  prove  to  be  prac- 
tical vacua  to  the  rays  of  heat;  for  every  ray,  or,  more 
strictly  speaking,  for  every  unit  of  wave -motion,  which 
any  one  of  them  intercepts,  perfectly  transparent  ammonia 
intercepts  5,460  units,  olefiant  gas  6,030  units,  while  sul- 
phurous acid  gas  absorbs  6,480  units.  What  becomes  of 
the  wave-motion  thus  intercepted?  It  is  applied  to  the 
heating  of  the  absorbing  gas.  Through  air,  oxygen,  hy- 
drogen, and  nitrogen,  the  waves  of  ether  pass  without  ab- 
sorption, and  these  gases  are  not  sensibly  changed  in  tem- 
perature by  the  most  powerful  calorific  rays.  The  position 
of  nitrous  oxide  in  the  foregoing  table  is  worthy  of  par- 
ticular notice.  In  this  gas  we  have  the  same  atoms  in  a 
state  of  chemical  union  that  exist  uncombined  in  the  at- 
mosphere; but  the  absorption  of  the  compound  is  1,800 
times  that  of  air. 

6.  Formation  of  Invisible  Foci 

This  extraordinary  deportment  of  the  elementary  gases 
naturally  directed  attention  to  elementary  bodies  in  other 
states  of  aggregation.  Some  of  Melloni*s  results  now  at- 
tained a  new  significance.  This  celebrated  experimenter 
had  found  crystals  of  sulphur  to  be  highly  pervious  to 
radiant  heat;    he  had  also  proved  that  lamp-black,  and 


46  .      FRAGMENTS    OF  SCIENCE 

black  glass  (which  owes  its  blackness  to  the  element  car- 
bon), were  to  a  considerable  extent  transparent  to  calorific 
rays  of  low  refrangibility.  These  facts,  harmonizing  so 
strikingly  with  the  deportment  of  the  simple  gases,  sug- 
gested further  inquiry.  Sulphur  dissolved  in  bisulphide 
of  carbon  was  found  almost  perfectly  diathermic.  The 
dense  and  deeply- colored  element  bromine  was  examined, 
and  found  competent  to  cut  off  the  light  of  our  most  bril- 
liant flames,  while  it  transmitted  the  invisible  calorific  rays 
with  extreme  freedom.  Iodine,  the  companion  element  of 
bromine,  was  next  thought  of,  but  it  was  found  impracti- 
cable to  examine  the  substance  in  its  usual  solid  condi- 
tion. It,  however,  dissolves  freely  in  bisulphide  of  car- 
bon. There  is  no  chemical  union  between  the  liquid  and 
the  iodine;  it  is  simply  a  case  of  solution,  in  which  the 
uncombined  atoms  of  the  element  can  act  upon  the  radi- 
ant heat.  When  permitted  to  do  so,  it  was  found  that  a 
layer  of  dissolved  iodine,  sufficiently  opaque  to  cut  off  the 
light  of  the  midday  sun,  was  almost  absolutely  transparent 
to  the  invisible  calorific  rays.^ 

By  prismatic  analysis  Sir  William  Herschel  separated 
the  luminous  from  the  non-luminous  rays  of  the  sun,  and 
he  also  sought  to  render  the  obscure  rays  visible  by  con- 
centration. Intercepting  the  luminous  portion  of  his  spec- 
trum, he  brought,  by  a  converging  lens,  the  ultra-red  rays 
to  a  focus,  but  by  this  condensation  he  obtained  no  light. 
The  solution  of  iodine  offers  a  means  of  filtering  the  solar 
beam,  or,  failing  it,  the  beam  of  the  electric  lamp,  which 


*  Professor  Dewar  has  recently  succeeded  in  producing  a  medium  highly 
opaque  to  light,  and  highly  transparent  to  obscure  heat,  by  fusing  together 
sulphur  and  iodine. 


RADIATION  47 

renders  attainable  far  more  powerful  foci  of  invisible  rays 
than  could  possibly  be  obtained  by  the  method  of  Sir  Wil- 
liam Herschel.  For  to  form  his  spectrum  he  was  obliged 
to  operate  upon  solar  light  which  had  passed  through  a 
narrow  slit  or  through  a  small  aperture,  the  amount  of  the 
obscure  heat  being  limited  by  this  circumstance.  But  with 
our  opaque  solution  we  may  employ  the  entire  surface  of 
the  largest  lens,  and  having  thus  converged  the  rays,  lumi- 
nous and  non-luminous,  we  can  intercept  the  former  by  the 
iodine,  and  do  what  we  please  with  the  latter.  Experi- 
ments of  this  character,  not  only  with  the  iodine  solution, 
but  also  with  black  glass  and  layers  of  lamp-black,  were 
publicly  performed  at  the  Eoyal  Institution  in  the  early 
part  of  1862,  and  the  effects  at  the  foci  of  invisible  rays, 
then  obtained,  were  such  as  had  never  been  witnessed 
previously. 

In  the  experiments  here  referred  to,  glass  lenses  were 
employed  to  concentrate  the  rays.  But  glass,  though 
highly  transparent  to  the  luminous,  is  in  a  high  degree 
opaque  to  the  invisible,  heat-rays  of  the  electric  lamp,  and 
hence  a  large  portion  of  those  rays  was  intercepted  by  the 
glass.  The  obvious  remedy  here  is  to  employ  rock-salt 
lenses  instead  of  glass  ones,  or  to  abandon  the  use  of 
lenses  wholly,  and  to  concentrate  the  rays  by  a  metallic 
mirror.  Both  of  these  improvements  have  been  intro- 
duced, and,  as  anticipated,  the  invisible  foci  have  been 
thereby  rendered  more  intense.  The  mode  of  operating 
remains,  however,  the  same,  in  principle,  as  that  made 
known  in  1862.  It  was  then  found  that  an  instant's  ex- 
posure of  the  face  of  the  thermo-electric  pile  to  the  focus 
of  invisible  rays,  dashed  the  needles  of  a  coarse  galvanom- 
eter violently  aside.     It  is  now  found  that  on  substituting 


48  FRAGMENTS   OF  SCIENCE 

for  the  face  of  the  thermo-electric  pile  a  combustible  body, 
the  invisible  rays  are  competent  to  set  that  body  on  fire. 

6.    Visible  and  Invisible  Bays  of  the  Electric  Light 

We  have  next  to  examine  what  proportion  the  non- 
luminous  rays  of  the  electric  light  bear  to  the  luminous 
ones.  This  the  opaque  solution  of  iodine  enables  us  to 
do  with  an  extremely  close  approximation  to  the  truth. 
The  pure  bisulphide  of  carbon,  which  is  the  solvent  of 
the  iodine,  is  perfectly  transparent  to  the  luminous,  and 
almost  perfectly  transparent  to  the  dark,  rays  of  the  elec- 
tric lamp.  Supposing  the  total  radiation  of  the  lamp  to 
pass  through  the  transparent  bisulphide,  while  through  the 
solution  of  iodine  only  the  dark  rays  are  transmitted.  If 
we  determine,  by  means  of  a  thermo-electric  pile,  the  total 
radiation,  and  deduct  from  it  the  purely  obscure,  we  ob- 
tain the  value  of  the  purely  luminous  emission.  Experi- 
ments, performed  in  this  way,  prove  that  if  all  the  visible 
rays  of  the  electric  light  were  converged  to  a  focus  of  daz- 
zling brilliancy,  its  heat  would  only  be  one-eighth  of  that 
produced  at  the  unseen  focus  of  the  invisible  rays. 

Exposing  his  thermometers  to  the  successive  colors  of 
the  solar  spectrum,  Sir  William  Herschel  determined  the 
heating  power  of  each,  and  also  that  of  the  region  beyond 
the  extreme  red.  Then  drawing  a  straight  line  to  repre- 
sent the  length  of  the  spectrum,  he  erected,  at  various 
points,  perpendiculars  to  represent  the  calorific  intensity 
existing  at  those  points.  Uniting  the  ends  of  all  his  per- 
pendiculars, he  obtained  a  curve  which  showed  at  a  glance 
the  manner  in  which  the  heat  was  distributed  in  the  solar 
spectrum.  Professor  Muller  of  Freiburg,  with  improved 
instruments,  afterward  made  similar  experiments,  and  con- 


RADIATION  49 

structed  a  more  accurate  diagram  of  tlie  same  kind.  We 
have  now  to  examine  the  distribution  of  heat  in  the  spec- 
trum of  the  electric  light;  and  for  this  purpose  we  shall 
employ  a  particular  form  of  the  thermo-electric  pile,  de- 
vised by  Melloni.  Its  face  is  a  rectangle,  which  by  means 
of  movable  side-pieces  can  be  rendered  as  narrow  as  de- 
sired. We  can,  for  example,  have  the  face  of  the  pile 
the  tenth,  the  hundredth,  or  even  the  thousandth,  of 
an  inch  in  breadth.  By  means  of  an  endless  screw, 
this  linear  thermo-electric  pile  may  be  moved  through 
the  entire  spectrum,  from  the  violet  to  the  red,  the 
amount  of  heat  falling  upon  the  pile  at  every  point  of 
its  march  being  declared  by  a  magnetic  needle  associated 
with  the  pile. 

When  this  instrument  is  brought  up  to  the  violet  end 
of  the  spectrum  of  the  electric  light,  the  heat  is  found  to 
be  insensible.  As  the  pile  is  gradually  moved  from  the 
violet  end  toward  the  red,  heat  soon  manifests  itself,  aug- 
menting as  we  approach  the  red.  Of  all  the  colors  of  the 
visible  spectrum  the  red  possesses  the  highest  heating 
power.  On  pushing  the  pile  into  the  dark  region  be- 
yond  the  red,  the  heat,  instead  of  vanishing,  rises  sud- 
denly and  enormously  in  intensity,  until  at  some  dis- 
tance beyond  the  red  it  attains  a  maximum.  Moving 
the  pile  still  forward,  the  thermal  power  falls  somewhat 
more  rapidly  than  it  rose.  It  then  gradually  shades 
away,  but,  for  a  distance  beyond  the  red  greater  than 
the  length  of  the  whole  visible  spectrum,  signs  of  heat 
may  be  detected. 

Drawing  a  datum  line,  and  erecting  along  it  perpen- 
diculars, proportional  in  length  to  the  thermal  intensity  at 
the  respective  points,  we  obtain  the  extraordinary  curve, 

SOIBNOB 3 


50  FRAGMENTS   OF  SCIENCE 

shown  on  the  opposite  page,  which  exhibits  the  distribu- 
tion of  heat  in  the  spectrum  of  the  electric  light.  In  the 
region  of  dark  rays,  beyond  the  red,  the  curve  shoots  up 
to  B,  in  a  steep  and  massive  peak — a  kind  of  Matterhorn 
of  heat,  which  dwarfs  the  portion  of  the  diagram  c  D  E, 
representing  the  luminous  radiation.  Indeed  the  idea 
forced  upon  the  mind  by  this  diagram  is  that  the  light 
rays  are  a  mere  insignificant  appendage  to  the  heat  rays 
represented  by  the  area  A  b  c  D,  thrown  in,  as  it  were,  by 
nature  for  the  purpose  of  vision. 

The  diagram  drawn  by  Professor  Miiller  to  represent 
the  distribution  of  heat  in  the  solar  spectrum  is  not  by  any 
means  so  striking  as  that  just  described,  and  the  reason, 
doubtless,  is  that  prior  to  reaching  the  earth  the  solar  rays 
have  to  traverse  our  atmosphere.  By  the  aqueous  vapor 
there  diffused,  the  summit  of  the  peak  representing  the 
sun's  invisible  radiation  is  cut  off.  A  similar  lowering  of 
the  mountain  of  invisible  heat  is  observed  when  the  rays 
from  the  electric  light  are  permitted  to  pass  through  a  film 
of  water,  which  acts  upon  them  as  the  atmospheric  vapor 
acts  upon  the  rays  of  the  sun. 

7.    Combustion  by  Invisible  Rays 

The  sun's  invisible  rays  far  transcend  the  visible  ones 
in  heating  power,  so  that  if  the  alleged  performances  of 
Archimedes  during  the  siege  of  Syracuse  had  any  founda- 
tion in  fact,  the  dark  solar  rays  would  have  been  the  phi- 
losopher's chief  agents  of  combustion.  On  a  small  scale 
we  can  readily  produce,  with  the  purely  invisible  rays  of 
the  electric  light,  all  that  Archimedes  is  said  to  have 
performed  with  the  sun's  total  radiation.  Placing  behind 
the   electric    light  a   small   concave   mirror,    the    rays    are 


RADIATION 


51 


52  FRAGMENTS   OF  SCIENCE 

converged,  the  cone  of  reflected  rays  and  their  point  of 
convergence  being  rendered  clearly  visible  by  the  dust  al- 
ways floating  in  the  air.  Placing  between  the  luminous 
focus  and  the  source  of  rays  our  solution  of  iodine,  the 
light  of  the  cone  is  entirely  cut  away;  but  the  intolerable 
heat  experienced  when  the  hand  is  placed,  even  for  a  mo- 
ment, at  the  dark  focus,  shows  that  the  calorific  rays  pass 
unimpeded  through  the  opaque  solution. 

Almost  anything  that  ordinary  fire  can  effect  may  be 
accomplished  at  the  focus  of  invisible  rays;  the  air  at  the 
focus  remaining  at  the  same  time  perfectly  cold,  on  account 
of  its  transparency  to  the  heat-rays.  An  air  thermometer, 
with  a  hollow  rock-salt  bulb,  would  be  unaffected  by  the 
heat  of  the  focus:  there  would  be  no  expansion,  and  in 
the  open  air  there  is  no  convection.  The  ether  at  the 
foe  as,  and  not  the  air,  is  the  substance  in  which  the  heat 
is  embodied.  A  block  of  wood,  placed  at  the  focus,  ab- 
sorbs the  heat,  and  dense  volumes  of  smoke  rise  swiftly 
upward,  showing  the  manner  in  which  the  air  itself  would 
rise,  if  the  invisible  rays  were  competent  to  heat  it.  At 
the  perfectly  dark  focus  dry  paper  is  instantly  inflamed: 
chips  of  wood  are  speedily  burned  up:  lead,  tin,  and  zinc 
are  fused:  and  disks  of  charred  paper  are  raised  to  vivid 
incandescence.  It  might  be  supposed  that  the  obscure  rays 
would  show  no  preference  for  black  over  white;  but  they 
do  show  a  preference,  and  to  obtain  rapid  combustion,  the 
body,  if  not  already  black,  ought  to  be  blackened.  When 
metals  are  to  be  burned,  it  is  necessary  to  blacken  or  other- 
wise tarnish  them,  so  as  to  diminish  their  reflective  power. 
Blackened  zinc  foil,  when  brought  into  the  focus  of  invis- 
ible rays,  is  instantly  caused  to  blaze,  and  burns  with  its 
peculiar  purple  light.      Magnesium  wire  flattened,   or  tar- 


RADIATION  53 

nished  magnesium  ribbon,  also  bursts  into  flame.  Pieces 
of  charcoal  suspended  in  a  receiver  full  of  oxygen  are  also 
set  on  £re  when  the  invisible  focus  falls  upon  them;  the 
dark  rays,  after  having  passed  through  the  receiver,  still 
possessing  sufficient  power  to  ignite  the  charcoal,  and  thus 
initiate  the  attack  of  the  oxygen.  If,  instead  of  being 
plunged  in  oxygen,  the  charcoal  be  suspended  in  vacuo, 
it  immediately  glows  at  the  place  where  the  focus  falls. 

8.   Transmutation  of  Bays  ;*   Calorescence 

Eminent  experimenters  were  long  occupied  in  demon- 
strating the  substantial  identity  of  light  and  radiant  heat, 
and  we  have  now  the  means  of  offering  a  new  and  striking 
proof  of  this  identity.  A  concave  mirror  produces,  beyond 
the  object  which  it  reflects,  an  inverted  and  magnified  im- 
age of  the  object.  Withdrawing,  for  example,  our  iodine 
solution,  an  intensely  luminous  inverted  image  of  the  car- 
bon points  of  the  electric  light  is  formed  at  the  focus  of 
the  mirror  employed  in  the  foregoing  experiments.  When 
the  solution  is  interposed,  and  the  light  is  cut  away,  what 
becomes  of  this  image?  It  disappears  from  sight;  but  an 
invisible  thermograph  remains,  and  it  is  only  the  peculiar 
constitution  of  our  eyes  that  disqualifies  us  from  seeing 
the  picture  formed  by  the  calorific  rays.  Falling  on  white 
paper,  the  image  chars  itself  out:  falling  on  black  paper, 
two  holes  are  pierced  in  it,  corresponding  to  the  images 
of  the  two  coke  points:  but  falling  on  a  thin  plate  of  car- 
bon in  vacuo,  or  upon  a  thin  sheet  of  platinized  platinum, 
either  in  vacuo  or  in  air,  radiant  heat  is  converted  into 
light,  and  the  image  stamps  itself  in  vivid  incandescence 

*  I  borrow  tills  term  from  Professor  Challis,  **Philo8(^^cal  Magazine/' 
Tol.  xii.  p.  621. 


54  FRAGMENTS   OF  SCIENCE 

upon  both  the  carbon  and  the  metal.  Results  similar  to 
those  obtained  with  the  electric  light  have  also  been 
obtained  with  the  invisible  rays  of  the  lime -light  and  of 
the  sun. 

Before  a  Cambridge  audience  it  is  hardly  necessary 
to  refer  to  the  e^Lcellent  researches  of  Professor  Stokes  at 
the  opposite  end  of  the  spectrum.  The  above  results  con- 
stitute a  kind  of  complement  to  his  discoveries.  Professor 
Stokes  named  the  phenomena  which  he  has  discovered 
and  investigated  Fluorescence;  for  the  new  phenomena 
here  described  I  have  proposed  the  term  Calorescence, 
He,  by  the  interposition  of  a  proper  medium,  so  lowered 
the  refrangibility  of  the  ultra-violet  rays  of  the  spectrum 
as  to  render  them  visible.  Here,  by  the  interposition  of 
the  platinum  foil,  the  refrangibility  of  the  ultra-red  rays 
is  so  exalted  as  to  render  them  visible.  Looking  through 
a  prism  at  the  incandescent  image  of  the  carbon  points, 
the  light  of  the  image  is  decomposed,  and  a  complete 
spectrum  is  obtained.  The  invisible  rays  of  the  electric 
light,  remolded  by  the  atoms  of  the  platinum,  shine  thus 
visibly  forth;  ultra-red  rays  being  converted  into  red, 
orange,  yellow,  green,  blue,  indigo,  violet,  and  ultra- 
violet ones.  Could  we,  moreover,  raise  the  original 
source  of  rays  to  a  sufficiently  high  temperature,  we  might 
not  only  obtain  from  the  dark  rays  of  such  a  source  a 
single  incandescent  image,  but  from  the  dark  rays  of  this 
image  we  might  obtain  a  second  one,  from  the  dark  rays 
of  the  second  a  third,  and  so  on — a  series  of  complete 
images  and  spectra  being  thus  extracted  from  the  invis- 
ible emission  of  the  primitive  source.* 

*  On  investigating  the  calorescence  produced  by  rays  transmitted  through 
glasses  of  variouL  coIots,  it  was  found  that  in  the  case  of  certain  specimens  of 


RADIATION  55 

9.  Deadness  of  the  Optic  Nerve  to  the  Calorific  Rays 

The  layer  of  iodine  used  in  the  foregoing  experiments 
intercepted  the  rays  of  the  noonday  sun.  No  trace  of 
light  from  the  electric  lamp  was  visible  in  the  darkest 
room,  even  when  a  white  screen  was  placed  at  the  focus 
of  the  mirror  employed  to  concentrate  the  light.  It  was 
thought,  however,  that  if  the  retina  itself  were  brought 
into  the  focus  the  sensation  of  light  might  be  experienced. 
The  danger  of  this  experiment  was  twofold.  If  the  dark 
rays  were  absorbed  in  a  high  degree  by  the  humors  of  the 
eye  the  albumen  of  the  humors  might  coagulate  along  the 
line  of  the  rays.  If,  on  the  contrary,  no  such  high  ab- 
sorption took  place,  the  rays  might  reach  the  retina  with 
a  force  sufficient  to  destroy  it.  To  test  the  likelihood  of 
these  results,  experiments  were  made  on  water  and  on  a 
solution  of  alum,  and  they  showed  it  to  be  very  improb-^ 
able  that  in  the  brief  time  requisite  for  an  experiment  any 
serious  damage  could  be  done.      The  eye  was  therefore 


blue  glass,  the  platinum  foil  glowed  with  a  pink  or  purplish  light.  The  effect 
was  not  subjective,  and  considerations  of  obvious  interest  are  suggested  by  it. 
Different  kinds  of  black  glass  differ  notably  as  to  their  power  of  transmitting 
radiant  heat.  When  thin,  some  descriptions  tint  the  sun  with  a  greenish  hue : 
others  make  it  appear  a  glowing  red  without  any  trace  of  green.  The  latter  are 
far  more  diathermic  than  the  former.  In  fact,  carbon  when  perfectly  dissolved 
and  incorporated  with  a  good  white  glass  is  highly  transparent  to  the  calorific 
rays,  and  by  employing  it  as  an  absorbent  the  phenomena  of  "calorescence" 
may  be  obtained,  though  in  a  less  striking  form  than  with  the  iodine.  The 
black  glass  chosen  for  thermometers,  and  intended  to  absorb  completely  the 
solar  heat,  may  entirely  fail  in  this  object,  if  the  glass  in  which  the  carbon  is 
incorporated  be  colorless.  To  render  the  bulb  of  a  thermometer  a  perfect 
absorbent,  the  glass  ought  in  the  first  instance  to  be  green.  Soon  after  the 
discovery  of  fluorescence  the  late  Dr.  William  Allen  Miller  pointed  to  the  lime- 
Kght  as  an  illustration  of  exalted  refrangibility.  Direct  experiments  have  since 
entirely  confirmed  the  view  expressed  at  p«^  210  of  his  work  on  "Chemistry," 
published  in  1856. 


56  FRAGMENTS   OF  SCIENCE 

caused  to  approach  the  dark  focus,  no  defence,  in  the  first 
instance,  being  provided;  but  the  heat,  acting  upon  the 
parts  surrounding  the  pupil,  could  not  be  borne.  An 
aperture  was  therefore  pierced  in  a  plate  of  metal,  and  the 
eye,  placed  behind  the  aperture,  was  caused  to  approach 
the  point  of  convergence  of  invisible  rays.  The  focus  was 
attained,  first  by  the  pupil  and  afterward  by  the  retina. 
Removing  the  eye,  but  permitting  the  plate  of  metal  to 
remain,  a  sheet  of  platinum  foil  was  placed  in  the  posi- 
tion occupied  by  the  retina  a  moment  before.  The  plati- 
num became  red-hot.  No  sensible  damage  was  done  to 
the  eye  by  this  experiment;  no  impression  of  light  was 
produced;  the  optic  nerve  was  not  even  conscious  of  heat. 
But  the  humors  of  the  eye  are  known  to  be  highly  im- 
pervious to  the  invisible  calorific  rays,  and  the  question 
therefore  arises,  **Did  the  radiation  in  the  foregoing  ex- 
•periment  reach  the  retina  at  all?*'  The  answer  is,  that 
the  rays  were  in  part  transmitted  to  the  retina,  ^nd  in  part 
absorbed  by  the  humors.  Experiments  on  th^  eye  of  an 
ox  showed  that  the  proportion  of  obscure  rays  which 
reached  the  retina  amounted  to  18  per  cent  of  the  total 
radiation;  while  the  luminous  emission  from  the  electric 
light  amounts  to  no  more  than  10  per  cent  of  the  same 
total.  Were  the  purely  luminous  rays  of  the  electric  lamp 
converged  by  our  mirror  to  a  focus,  there  can  be  no  doubt 
as  to  the  fate  of  a  retina  placed  there.  Its  ruin  would  be 
inevitable;  and  yet  this  would  be  accomplished  by  an 
amount  of  wave-motion  but  little  more  than  half  of  that 
which  the  retina,  without  exciting  consciousness,  bears  at 
the  focus  of  invisible  rays. 

This  subject  will  repay  a  moment's  further  attention. 
At  a  common  distance  of  a  foot  the  visible  radiation  of 


RADIATION  57 

the  electric  light  employed  in  these  experiments  is  800 

times  the  light  of  a  candle.  At  the  same  distance,  the 
portion  of  the  radiation  of  the  electric  light  which  reaches 
the  retina,  but  fails  to  excite  vision,  is  about  1,500  times 
the  luminous  radiation  of  the  candle.*  But  a  candle  on  a 
clear  night  can  readily  be  seen  at  a  distance  of  a  mile,  its 
light  at  this  distance  being  less  than  Mjawjm  of  its  light  at 
the  distance  of  a  foot.  Hence,  to  make  the  candle-light 
a  mile  off  equal  in  power  to  the  non-luminous  radiation 
received  from  the  electric  light  at  a  foot  distance,  its  in- 
tensity would  have  to  be  multiplied  by  1,500  x  20,000,000, 
or  by  thirty  thousand  millions.  Thus  the  thirty  thousand 
millionth  part  of  the  invisible  radiation  from  the  electric 
light,  received  by  the  retina  at  the  distance  of  a  foot, 
would,  if  slightly  changed  in  character,  be  amply  suffi- 
cient to  provoke  vision.  Nothing  could  more  forcibly 
illustrate  that  special  relationship  supposed  by  Melloni 
and  others  to  subsist  between  the  optic  nerve  and  the 
oscillating  periods  of  luminous  bodies.  The  optic  nerve 
responds,  as  it  were,  to  the  waves  with  which  it  is  in  con- 
sonance, while  it  refuses  to  be  excited  by  others  of  almost 
infinitely  greater  energy,  whose  periods  of  recurrence  are 
not  in  unison  with  its  own. 

10.  Persistence  of  Rays 

At  an  early  part  of  this  lecture  it  was  affirmed  that 
when  a  platinum  wire  was  gradually  raised  to  a  state  of 
high  incandescence,  new  rays  were  constantly  added,  while 


*  It  will  be  borne  in  mind  that  the  heat  which  any  ray,  luminous  or  non- 
luminous,  is  competent  to  generate  is  the  true  measure  of  the  energy  of 
the  ray. 


68  FRAGMENTS   OF  SCIENCE 

the  intensity  of  the  old  ones  was  increased.  Thus,  in  Dr. 
Draper's  experiments,  the  rise  of  temperature  that  gener- 
ated the  orange,  yellow,  green,  and  blue,  augmented  the 
intensity  of  the  red.  What  is  true  of  the  red  is  true  of 
every  other  ray  of  the  spectrum,  visible  and  invisible. 
We  cannot  indeed  see  the  augmentation  of  intensity  in 
the  region  beyond  the  red,  but  we  can  measure  it  and  ex- 
press it  numerically. 

With  this  view  the  following  experiment  was  per- 
formed: A  spiral  of  platinum  wire  was  surrounded  by 
a  small  glass  globe  to  protect  it  from  currents  of  air; 
through  an  orifice  in  the  globe  the  rays  could  pass  from 
the  spiral  and  fall  afterward  upon  a  thermo-electric  pile. 
Placing  in  front  of  the  orifice  an  opaque  solution  of 
iodine,  the  platinum  was  gradually  raised  from  a  low, 
dark  heat  to  the  fullest  incandescence,  with  the  follow- 
ing results: 

AppearaxKse  Ener^of 

of  spiral  obscure  radiatkm 

Dark 1 

Bark,  but  hot4er .        .        • 8 

Bark,  but  still  hotter 5 

Bark,  but  stiU  hotter 10 

Feeble  red 19 

Dull  red 36 

Bed .  87 

FuUred 62 

Orange 89 

Bright  orange 144 

Yellow 202 

White 276 

Intense  white      , 440 

Thus  the  augmentation  of  the  electric  current,  which 
raises  the  wire  from  its  primitive  dark  condition  to  an  in- 
tense white  heat,  exalts  at  the  same  time  the  energy  of  the 


RADIATION  69 

obscure  radiation,   until  at  the  end  it  is  fully  440  times 
what  it  was  at  the  beginning. 

What  has  been  here  proved  true  of  the  totality  of  the 
ultra-red  rays  is  true  for  each  of  them  singly.  Placing 
our  linear  thermo-electric  pile  in  any  part  of  the  ultra-red 
spectrum,  it  may  be  proved  that  a  ray  once  emitted  con- 
tinues to  be  emitted  with  increased  energy  as  the  temper- 
ature is  augmented.  The  platinum  spiral,  so  often  referred 
to,  being  raised  to  whiteness  by  an  electric  current,  a  bril- 
liant spectrum  was  formed  from  its  light.  A  linear  thermo- 
electric pile  was  placed  in  the  region  of  obscure  rays  be- 
yond the  red,  and  by  diminishing  the  current  the  spiral 
was  reduced  to  a  low  temperature.  It  was  then  caused  to 
pass  through  various  degrees  of  darkness  and  incandes- 
cence, with  the  following  results: 


Appearance  Energy  of 

of  spiral  obscure  rays 

Dark 1 

Dark 6 

Faint  red 10 

DuUred 13 

Red 18 

Full  red 27 

Orange 60 

Yellow 93 

White 122 


Here,  as  in  the  former  case,  the  dark  and  bright  radia- 
tions reached  their  maximum  together;  as  the  one  aug- 
mented, the  other  augmented,  until  at  last  the  energy  of 
the  obscure  rays  of  the  particular  refrangibility  here 
chosen  became  122  times  what  it  was  at  first.  To  reach 
a  white  heat  the  wire  has  to  pass  through  all  the  stages 
of  invisible  radiation,  but  in  its  most  brilliant  condition 


<50  FRAGMENTS   OF  SCIENCE 

it  embraces,  in  an  intensified  form,  the  rays  of  all  those 
stages. 

And  thus  it  is  with  all  other  kinds  of  matter,  as  far  as 
they  have  hitherto  been  examined.  Coke,  whether  brought 
to  a  white  heat  by  the  electric  current,  or  by  the  oxyhydro- 
gen  jet,  pours  out  invisible  rays  with  augmented  energy,  as 
its  light  is  increased.  The  same  is  true  of  lime,  bricks, 
and  other  substances.  It  is  true  of  all  metals  which  are 
capable  of  being  heated  to  incandescence.  It  also  holds 
good  for  phosphorus  burning  in  oxygen.  Every  gush  of 
dazzling  light  has  associated  with  it  a  gush  of  invisible 
radiant  heat,  which  far  transcends  the  light  in  energy. 
This  condition  of  things  applies  to  all  bodies  capable  of 
being  raised  to  a  white  heat,  either  in  the  solid  or  the 
molten  condition.  It  would  doubtless  also  apply  to  the 
luminous  fogs  formed  by  the  condensation  of  incandescent 
vapors.  In  such  cases  when  the  curve  representing  the 
radiant  energy  of  the  body  is  constructed,  the  obscure  ra- 
diation towers  upward  like  a  mountain,  the  luminous  radi- 
ation resembling  a  mere  "spur'*  at  its  base.  From  the 
very  brightness  of  the  light  of  some  of  the  fixed  stars  we 
may  infer  the  intensity  of  that  dark  radiation,  which  is 
the  precursor  and  inseparable  associate  of  their  luminous 
rays. 

We  thus  find  the  luminous  radiation  appearing  when 
the  radiant  body  has  attained  a  certain  temperature;  or,  in 
other  words,  when  the  vibrating  atoms  of  the  body  have 
attained  a  certain  width  of  swing.  In  solid  and  molten 
bodies  a  certain  amplitude  cannot  be  surpassed  without 
the  introduction  of  periods  of  vibration,  which  provoke 
the  sense  of  vision.  How  are  we  to  figure  this?  If  per- 
mitted to  speculate,  we  might  ask,  Are  not  these  more  rapid 


RADIATION  61 

vibrations  the  progeny  of  the  slower  ?  Is  it  not  really  the 
mutual  action  of  the  atoms,  when  they  swing  through  very 
wide  spaces,  and  thus  encroach  upon  each  other,  that 
causes  them  to  tremble  in  quicker  periods?  If  so,  what- 
ever be  the  agency  by  which  the  large  swinging  space  is 
obtained,  we  shall  have  light-giving  vibrations  associated 
with  it.  It  matters  not  whether  the  large  amplitudes  be 
produced  by  the  strokes  of  a  hammer,  or  by  the  blows  of 
the  molecules  of  a  non-luminous  gas,  like  air  at  some 
height  above  a  gas-flame;  or  by  the  shock  of  the  ether 
particles  when  transmitting  radiant  heat.  The  result  in 
all  cases  will  be  incandescence.  Thus,  the  invisible  waves 
of  our  filtered  electric  beam  may  be  regarded  as  generating 
synchronous  vibrations  among  the  atoms  of  the  platinum 
on  which  they  impinge;  but,  once  these  vibrations  have 
attained  a  certain  amplitude,  the  mutual  jostling  of  the 
atoms  produces  quicker  tremors,  and  the  light-giving 
waves  follow  as  the  necessary  product  of  the  heat-giv- 
ing ones. 

11.  Absorption  of  Radiant  Heat  by  Vapors  and  Odors 

We  commenced  the  demonstrations  brought  forward  in 
^this  lecture  by  experiments  on  permanent  gases,  and  we 
have  now  to  turn  our  attention  to  the  vapors  of  vola- 
tile liquids.  Here,  as  in  the  case  of  the  gases,  vast 
differences  have  been  proved  to  exist  between  various 
kinds  of  molecules,  as  regards  their  power  of  intercept- 
ing the  calorific  waves.  While  some  vapors  allow  the 
waves  a  comparatively  free  passage,  the  faintest  mix- 
ture of  other  vapors  causes  a  deflection  of  the  magnetic 
needle.  Assuming  the  absorption  effected  by  air,  at  a 
pressure  of  one  atmosphere,  to   be  unity,   the   following 


C2  FRAGMENTS   OF  SCIENCE 

are   the  absorptions  effected  by  a  series  of  vapors  at 
pressure  of  ^th  of  an  atmosphere: 

Name  of  vapor  AbsOTption 

Bisulphide  of  carbon 47 

Iodide  of  methyl 115 

Benzol 136 

Amylene 321 

Sulphuric  ether .  440 

Formic  ether 548 

Acetic  ether 612 


Bisulphide  of  carbon  is  the  most  transparent  vapor  in 
this  list;  and  acetic  ether  the  most  opaque;  i?ffth  of  an  at- 
mosphere of  the  former,  however,  produces  47  times  the 
effect  of  a  whole  atmosphere  of  air,  while  ikth.  of  an  atmos- 
phere of  the  latter  produces  612  times  the  effect  of  a  whole 
atmosphere  of  air.  Eeducing  dry  air  to  the  pressure  of  the 
acetic  ether  here  employed,  and  comparing  them  then  to- 
gether, the  quantity  of  wave-motion  intercepted  by  the 
ether  would  be  many  thousand  times  that  intercepted  by 
the  air. 

Any  one  of  these  vapors  discharged  into  the  free 
atmosphere,  in  front  of  a  body  emitting  obscure  rays, 
intercepts  more  or  less  of  the  radiation.  A  similar  effect 
is  produced  by  perfumes  diffused  in  the  air,  though  their 
attenuation  is  known  to  be  almost  infinite.  Carrying, 
for  example,  a  current  of  dry  air  over  bibulous  paper, 
moistened  by  patchouli,  the  scent  taken  up  by  the  cur« 
rent  absorbs  30  times  the  quantity  of  heat  intercepted 
by  the  air  which  carries  it;  and  yet  patchouli  acts  more 
feebly  on  radiant  heat  than  any  other  perfume  yet  ex- 
amined. Here  follow  the  results  obtained  with  various 
essential  oils,   the  odor,   in  each   case,   being  carried  by 


RADIATION  63 

a  current  of  dry  air  into  the  tube  already  employed  for 
gases  and  vapors: 

Name  of  perfbnw  AbaorptioD 

Patchouli     .••••••..       30 

Sandal  wood        •••••••.32 

Geranium    ..•••••..       33 

Oil  of  cloves 34 

Otto  of  roses        ••••••••37 

Bergamot    .         • 44 

Neroli 47 

Lavender     ••••60 

Lemon  .••••«••         .66 

Portugal      .••••«•••      67 

Thyme 68 

Rosemary    .        •••••••.74 

Oil  of  laurel 80 

Camomile  flowers 87 

Cassia •        •        .     109 

Spikenard    .••••••••     356 

Aniseed 372 

Thus  the  absorption  by  a  tube  lull  of  dry  air  being  1, 
that  of  the  odor  of  patchouli  diffused  in  it  is  SO,  that  of 
lavender  60,  that  of  rosemary  74,  while  that  of  aniseed 
amounts  to  872.  It  would  be  idle  to  speculate  on  the 
quantities  of  matter  concerned  in  these  actions. 

12.  Aqueous  Vapor  in  relation  to  the  Terrestrial 

Temperatures 

We  are  now  fully  prepared  for  a  result  which,  without 
such  preparation,  might  appear  incredible.  Water  is,  to 
some  extent,  a  volatile  body,  and  our  atmosphere,  resting 
as  it  does  upon  the  surface  of  the  ocean,  receives  trom  it 
a  continual  supply  of  aqueous  vapor.  It  would  be  an 
error  to  confound  clouds  or  fog,  or  any  visible  mist,  with 
the  vapor  of  water,  which  ia  a  perfectly  impalpable  gas, 


64  FRAGMENTS    OF  SCIENCE 

diffused,  even  on  the  clearest  days,  throughout  the  atmos- 
phere. Compared  with  the  great  body  of  the  air,  the  aque- 
ous vapor  it  contains  is  of  almost  infinitesimal  amount,  99J^ 
out  of  every  100  parts  of  the  atmosphere  being  composed 
of  oxygen  and  nitrogen.  In  the  absence  of  experiment, 
we  should  never  think  of  ascribing  to  this  scant  and  vary- 
ing constituent  any  important  influence  on  terrestrial  radi- 
ation; and  yet  its  influence  is  far  more  potent  than  that 
of  the  great  body  of  the  air.  To  say  that  on  a  day  of 
average  humidity  in  England,  the  atmospheric  vapor  ex- 
erts 100  times  the  action  of  the  air  itself,  would  certainly 
be  an  understatement  of  the  fact.  Comparing  a  single 
molecule  of  aqueous  vapor  with  an  atom  of  either  of  the 
main  constituents  of  our  atmosphere,  I  am  not  prepared  to 
say  how  many  thousand  times  the  action  of  the  former 
exceeds  that  of  the  latter. 

But  it  must  be  borne  in  mind  that  these  large  numbers 
depend,  in  part,  on  the  extreme  feebleness  of  the  air;  the 
power  of  aqueous  vapor  seems  vast,  because  that  of  the  air 
with  which  it  is  compared  is  infinitesimal.  Absolutely 
considered,  however,  this  substance,  notwithstanding  its 
small  specific  gravity,  exercises  a  very  potent  action. 
Probably  from  10  to  15  per  cent  of  the  heat  radiated 
from  the  earth  is  absorbed  within  10  or  20  feet  of  the 
earth's  surface.  This  must  evidently  be  of  the  utmost 
consequence  to  the  life  of  the  world.  Imagine  the  super- 
ficial molecules  of  the  earth  agitated  with  the  motion  of 
heat,  and  imparting  it  to  the  surrounding  ether;  this  mo- 
tion would  be  carried  rapidly  away,  and  lost  forever  to 
our  planet,  if  the  waves  of  ether  had  nothing  but  the  air 
to  contend  with  in  their  outward  course.  But  the  aqueous 
vapor  takes  up  the  motion,  and  becomes  thereby  heated, 


RADIATION  65 

thus  wrapping  the  earth  like  a  warm  garment,  and  pro- 
tecting  its  surface  from  the  deadly  chill  which  it  would 
otherwise  sustain.  Various  philosophers  have  speculated 
on  the  influence  of  an  atmospheric  envelope.  De  Saus- 
sure,  Fourier,  M.  Pouillet,  and  Mr.  Hopkins  have,  one 
and  all,  enriched  scientific  literature  with  contributions  on 
this  subject,  but  the  considerations  which  these  eminent 
men  have  applied  to  atmospheric  air,  have,  if  my  experi- 
ments be  correct,  to  be  transferred  to  the  aqueous  vapor. 
The  observations  of  meteorologists  furnish  important, 
though  hitherto  unconscious,  evidence  of  the  influence  of 
this  agent  Wherever  the  air  is  dry  we  are  liable  to  daily 
extremes  of  temperature.  By  day,  in  such  places,  the  sun's 
heat  reaches  the  earth  unimpeded,  and  renders  the  maxi- 
mum high;  by  night,  on  the  other  hand,  the  earth's  heat 
escapes  unhindered  into  space,  and  renders  the  minimum 
low.  Hence  the  difference  between  the  maximum  and  min- 
imum is  greatest  where  the  air  is  driest  In  the  plains  of 
India,  on  the  heights  of  the  Himalaya,  in  Central  Asia,  in 
Australia — wherever  drought  reigns,  we  have  the  heat  of 
day  forcibly  contrasted  with  the  chill  of  night  In  the 
Sahara  itself,  when  the  sun's  rays  cease  to  impinge  on  the 
burning  soil,  the  temperature  runs  rapidly  down  to  freez- 
ing, because  there  is  no  vapor  overhead  to  check  the  cal- 
orific drain.  And  here  another  instance  might  be  added 
to  the  numbers  already  known,  in  which  nature  tends,  as 
it  were,  to  check  her  own  excess.  By  nocturnal  refrigera- 
tion, the  aqueous  vapor  of  the  air  is  condensed  to  water 
on  the  surface  of  the  earth;  and,  as  only  the  superficial 
portions  radiate,  the  act  of  condensation  makes  water  the 
radiating  body.  Now,  experiment  proves  that  to  the  rays 
emitted   by   water,    aqueous  vapor   is   especially   opaque. 


66  FRAGMENTS   OF  SCIENCE 

Hence  the  very  act  of  condensation,  consequent  on  ter- 
restrial cooling,  becomes  a  safeguard  to  the  earth,  impart- 
ing to  its  radiation  that  particular  character  which  renders 
it  most  liable  to  be  prevented  from  escaping  into  space. 

It  might,  however,  be  urged  that,  inasmuch  as  we  de- 
rive all  our  heat  from  the  sun,  the  self -same  covering  which 
protects  the  earth  from  chill  must  also  shut  out  the  solar 
radiation.  This  is  partially  true,  but  only  partially;  the 
sun's  rays  are  different  in  quality  from  the  earth's  rays, 
and  it  does  not  at  all  follow  that  the  substance  which  ab- 
sorbs the  one  must  necessarily  absorb  the  other.  Through 
a  layer  of  water,  for  example,  one-tenth  of  an  inch  in 
thickness,  the  sun*s  rays  are  transmitted  with  comparative 
freedom;  but  through  a  layer  half  this  thickness,  as  Mel- 
loni  has  proved,  no  single  ray  from  the  warmed  earth  could 
pass.  In  like  manner,  the  sun's  rays  pass  with  compara- 
tive freedom  through  the  aqueous  vapor  of  the  air:  the 
absorbing  power  of  this  substance  being  mainly  exerted 
upon  the  invisible  heat  that  endeavors  to  escape  from  the 
earth.  In  consequence  of  this  differential  action  upon  solar 
and  terrestrial  heat,  the  mean  temperature  of  our  planet  is 
higher  than  is  due  to  its  distance  from  the  sun. 

18.   Liquids  and  their    Vapors  in   relation  to  Radiant   Heat 

The  deportment  here  assigned  to  atmospheric  vapor  has 
been  established  by  direct  experiments  on  air  taken  from 
the  streets  and  parks  of  London,  from  the  downs  of  Epsom, 
from  the  hills  and  sea-beach  of  the  Isle  of  Wight,  and  also 
by  experiments  on  air,  in  the  first  instance  dried,  and  after- 
ward rendered  artificially  humid  by  pure  distilled  water. 
It  has  also  been  established  in  the  following  way:  Ten 
volatile  liquids  were  taken  at  random,  and  the  power  of 


RADIATION  67 

these  liquids,  at  a  common  thickness,  to  intercept  the 
waves  of  heat,  was  carefully  determined.  The  vapors  of 
the  liquids  were  next  taken,  in  quantities  proportional  to 
the  quantities  of  liquid,  and  the  power  of  the  vapors  to  in- 
tercept the  waves  of  heat  was  also  determined.  Commenc- 
ing with  the  substance  which  exerted  the  least  absorptive 
power,  and  proceeding  onward  to  the  most  energetic,  the 
following  order  of  absorption  was  observed: 

Liquids  Vapors 

Bisulphide  of  carbon.  Bisulphide  of  carbon. 

Chloroform.  Chloroform. 

Iodide  of  methyL  Iodide  of  methyl. 

Iodide  of  ethyl.  Iodide  of  ethyL 

Benzol.  Benzol. 

Amylene.  Amylene. 

Sulphuric  ether.  Sulphuric  ether. 

Acetic  ether.  Acetic  ether. 

Formic  ether.  Formic  ether. 

Alcohol  Alcohol. 
"Water. 

We  here  find  the  order  of  absorption  in  both  cases  to 
be  the  same.  We  have  liberated  the  molecules  from  the 
bonds  which  trammel  them  more  or  less  in  a  liquid  con- 
dition; but  this  change  in  their  state  of  aggregation  does 
not  change  their  relative  powers  of  absorption.  Kothing 
could  more  clearly  prove  that  the  act  of  absorption  de- 
pends upon  the  individual  molecule,  which  equally  asserts 
its  power  in  the  liquid  and  the  gaseous  state.  We  may 
safely  conclude  from  the  above  table  that  the  position  of 
a  vapor  is  determined  by  that  of  its  liquid.  Now,  at  the 
very  foot  of  the  list  of  liquids  stands  watery  signalizing  it- 
self above  all  others  by  its  enormous  power  of  absorption. 
And  from  this  fact,  even  if  no  direct  experiment  on  the 
vapor  of  water  had  ever  been  made,  we  should  be  entitled 


68  FRAGMENTS   OF  SCIENCE 

to  rank  tliat  vapor  as  our  most  powerful  absorber  of  radi- 
ant keat.  Its  attenuation,  however,  diminishes  its  action, 
I  have  proved  that  a  shell  of  air  two  inches  in  thickness 
surrounding  our  planet,  and  saturated  with  the  vapor  of 
sulphuric  ether,  would  intercept  36  per  cent  of  the  earth's 
radiation.  And,  though  the  quantity  of  aqueous  vapor 
necessary  to  saturate  air  is  much  less  than  the  amount  of 
sulphuric  ether  vapor  which  it  can  sustain,  it  is  still  ex- 
tremely probable  that  the  estimate  already  made  of  the 
action  of  atmospheric  vapor  within  10  feet  of  the  earth's 
surface,  is  under  the  mark;  and  that  we  are  indebted  to 
this  wonderful  substance,  to  an  extent  not  accurately  de- 
termined, but  certainly  far  beyond  what  has  hitherto  been 
imagined,  for  the  temperature  now  existing  at  the  surface 
of  the  globe. 

14.  Beciprocity  of  Radiation  and  Absorption 

Throughout  the  reflections  which  have  hitherto  occupied 
us,  the  image  before  the  mind  has  been  that  of  a  radiant 
source  sending  forth  calorific  waves,  which,  on  passing 
among  the  molecules  of  a  gas  or  vapor,  were  intercepted 
by  those  molecules  in  various  degrees.  In  all  cases  it  was 
the  transference  of  motion  from  the  ether  to  the  compara- 
tively quiescent  molecules  of  the  gas  or  vapor  that  occu- 
pied our  thoughts.  We  have  now  to  change  the  form  of 
our  conception,  and  to  figure  these  molecules  not  as  ab- 
sorbers, but  as  radiators,  not  as  the  recipients,  but  as  the 
originators  of  wave-motion.  That  is  to  say,  we  must  figure 
them  vibrating,  and  generating  in  the  surrounding  ether 
undulations  which  speed  through  it  with  the  velocity  of 
light.  Our  object  now  is  to  inquire  whether  the  act  of 
chemical  combination,  which  proves  so  potent  as  regards 


RADIATION  69 

the  phenomena  of  absorption,  does  not  also  manifest  its 
power  in  the  phenomena  of  radiation.  For  the  exami- 
nation of  this  question  it  is  necessary,  in  the  first  place, 
to  heat  onr  gases  and  vapors  to  the  same  temperature, 
and  then  examine  their  power  of  discharging  the  motion 
thus  imparted  to  them  upon  the  ether  in  which  they 
swing. 

A  heated  copper  ball  was  placed  above  a  ring  gas- 
burner  possessing  a  great  number  of  small  apertures,  the 
burner  being  connected  by  a  tube  with  vessels  containing 
the  various  gases  to  be  examined.  By  gentle  pressure  the 
gases  were  forced  through  the  orifices  of  the  burner  against 
the  copper  ball,  where  each  of  them,  being  heated,  rose  in 
an  ascending  column.  A  thermo-electrio  pile,  entirely 
screened  from  the  hot  ball,  was  exposed  to  the  radia- 
tion of  the  warm  gas,  while  the  deflection  of  a  magnetic 
needle  connected  with  the  pile  declared  the  energy  of  the 
radiation. 

By  this  mode  of  experhnent  it  was  proved  that  the  self- 
same molecular  arrangement,  which  renders  a  gas  a  power- 
ful absorber,  renders  it  a  powerful  radiator — ^that  the  atom 
or  molecule  which  is  competent  to  intercept  the  calorific 
waves  is,  in  the  same  degree,  competent  to  send  them  forth. 
Thus,  while  the  atoms  of  elementary  gases  proved  them- 
selves unable  to  emit  any  sensible  amount  of  radiant  heat, 
the  molecules  of  compound  gases  were  shown  to  be  capable 
of  powerfully  disturbing  the  surrounding  ether.  By  spe- 
cial modes  of  experiment  the  same  was  proved  to  hold 
good  for  the  vapors  of  volatile  liquids,  the  radiative  power 
of  every  vapor  being  found  proportional  to  its  absorptive 
power. 

The  method  of  experiment  here  pursued,  though  not  of 


to  FRAGMENTS   OF  SCIENCE 

the  simplest  character,  is  still  easy  to  grasp.  When  air  is 
permitted  to  rush  into  an  exhausted  tube,  the  temperature 
of  the  air  is  raised  to  a  degree  equivalent  to  the  vis  viva 
extinguished.*  Such  air  is  said  to  be  dynamically  heated, 
and,  if  pure,  it  shows  itself  incompetent  to  radiate,  even 
when  a  rock-salt  window  is  provided  for  the  passage  of  its 
rays.  But  if  instead  of  being  empty  the  tube  contain  a 
small  quantity  of  vapor,  the  warmed  air  communicates  its 
heat  by  contact  to  the  vapor,  the  molecules  of  which  con- 
vert into  the  radiant  form  the  heat  imparted  to  them  by 
the  atoms  of  the  air.  By  this  process  also,  which  I  have 
called  Dynamic  Badiation,  the  reciprocity  of  radiation  and 
absorption  has  been  conclusively  proved.* 

In  the  excellent  researches  of  Leslie,  De  la  Provostaye 
and  Desains,  and  Balfour  Stewart,  the  same  reciprocity,  as 
regards  solid  bodies,  haB  been  variously  illustrated;  while 
the  labors,  theoretical  and  experimental,  of  KirchhofE  have 
given  this  subject  a  wonderful  expansion,  and  enriched 
it  by  applications  of  the  highest  kind.  To  their  results 
are  now  to  be  added  the  foregoing,  whereby  gases  and 
vapors,  which  have  been  hitherto  thought  inaccessible  to 
experiments  with  the  thermo-electric  pile,  are  proved  by 
it  to  exhibit  the  indissoluble  duality  of  radiation  and 
absorption,  the  influence  of  chemical  combination  on 
both  being  exhibited  in  the  most  decisive  and  extraor- 
dinary way. 


*  See  page  19  for  a  definition  of  vis  viva. 

*  When  heated  air  imparts  its  motion  to  another  gas  or  vapor,  the  trans- 
ference  of  heat  is  accompanied  by  a  change  of  vibrating  period.  The 
Dynamic  Badiation  of  vapors  is  rendered  possible  by  this  transmatetion  id 
vibrations. 


^ 


BADIATIOH  Tl 


15.  Influence  of  Vibrating  Period    and  Molecular  Form, 

Physical  Analysis  of  the  Human  Breath 

In  the  foregoing  experiments  with  gases  and  vapors  we 
have  employed  throughout  invisible  rays,  and  found  some 
of  these  bodies  so  impervious  to  radiant  heat,  that  in 
lengths  of  a  few  feet  they  intercept  every  ray  as  effect- 
ually as  a  layer  of  pitch.  The  substances,  however,  which 
show  themselves  thus  opaque  to  radiant  heat  are  perfectly 
transparent  to  light.  Now,  the  rays  of  light  differ  from 
those  of  invisible  heat  merely  in  point  of  period,  the  for- 
mer failing  to  affect  the  retina  because  their  periods  of 
recurrence  are  too  slow.  Hence,  in  some  way  or  other, 
the  transparency  of  our  gases  and  vapors  depends  upon  the 
periods  of  the  waves  which  impinge  upon  them.  What  is 
the  nature  of  this  dependence?  The  admirable  researches 
of  Kirchhoff  help  us  to  an  answer.  The  atoms  and  mole- 
cules of  every  gas  have  certain  definite  rates  of  oscillation, 
and  those  waves  of  ether  are  most  copiously  absorbed 
whose  periods  of  recurrence  synchronize  with  those  of  the 
atomic  groups  among  which  they  pass.  Thus,  when  we 
find  the  invisible  rays  absorbed  and  the  visible  ones  trans- 
mitted by  a  layer  of  gas,  we  conclude  that  the  oscillating 
periods  of  the  atoms  constituting  the  gaseous  molecules 
coincide  with  those  of  the  invisible,  and  not  with  those 
of  the  visible,  spectrum. 

It  requires  some  discipline  of  the  imagination  to  form 
a  clear  picture  of  this  process.  Such  a  picture  is,  however, 
possible,  and  ought  to  be  obtained.  When  the  waves  of 
ether  impinge  upon  molecules  whose  periods  of  vibration 
coincide  with  the  recurrence  of  the  undulations,  the  timed 


72  FRAGMENTS  OF  SCIENCE 

strokes  of  the  waves  augment  the  vibration  of  the  mole* 
cules,  as  a  heavy  pendulum  is  set  in  motion  by  well-timed 
puffs  of  breath.  Millions  of  millions  of  shocks  are  received 
every  second  from  the  calorific  waves;  and  it  is  not  diffi- 
cult to  see  that  as  every  wave  arrives  just  in  time  to  repeat 
the  action  of  its  predecessor,  the  molecules  must  finally  be 
caused  to  swing  through  wider  spaces  than  if  the  arrivals 
were  not  so  timed.  In  fact,  it  is  not  difficult  to  see  that 
an  assemblage  of  molecules,  operated  upon  by  contending 
waves,  might  remain  practically  quiescent.  This  is  act- 
ually the  case  when  the  waves  of  the  visible  spectrum 
pass  through  a  transparent  gas  or  vapor.  There  is  here 
no  sensible  transference  of  motion  from  the  ether  to  the 
molecules;  in  other  words,  there  is  no  sensible  absorption 
of  heat. 

One  striking  example  of  the  influence  of  period  may  be 
here  recorded.  Carbonic  acid  gas  is  one  of  the  feeblest 
absorbers  of  the  radiant  heat  emitted  by  solid  bodies.  It 
is,  for  example,  to  a  great  extent  transparent  to  the  rays 
emitted  by  the  heated  copper  plate  already  referred  to. 
There  are,  however,  certain  rays,  comparatively  few  in 
number,  emitted  by  the  copper,  to  which  the  carbonic  acid 
is  impervious;  and  could  we  obtain  a  source  of  heat  emit- 
ting such  rays  only,  we  should  find  carbonic  acid  more 
opaque  to  the  radiation  from  that  source  than  any  other 
gas.  Such  a  source  is  actually  found  in  the  flame  of  car- 
bonic oxide,  where  hot  carbonic  acid  constitutes  the  main 
radiating  body.  Of  the  rays  emitted  by  our  heated  plate 
of  copper,  olefiant  gas  absorbs  ten  times  the  quantity  ab- 
sorbed by  carbonic  acid.  Of  the  rays  emitted  by  a  car- 
bonic oxide  flame,  carbonic  acid  absorbs  twice  as  much 
as  olefiant  gas.     This  wonderful  change  in  the  power  of 


RADIATION  73 

the  former,  as  an  absorber,  is  simply  due  to  the  fact  that 
the  periods  of  the  hot  and  cold  carbonic  acid  are  identi- 
cal, and  that  the  waves  from  the  flame  freely  transfer  their 
motion  to  the  molecules  which  synchronize  with  them. 
Thus  it  is  that  the  tenth  ot  an  atmosphere  of  carbonic 
acid,  enclosed  in  a  tube  four  feet  long,  absorbs  60  per 
cent  of  the  radiation  from  a  carbonic  oxide  flame,  while 
one-thirtieth  of  an  atmosphere  absorbs  48  per  cent  of  the 
heat  from  the  same  source. 

In  fact,  the  presence  of  the  minutest  quantity  of  car- 
bonic acid  may  be  detected  by  its  action  on  the  rays  from 
the  carbonic  oxide  flame.  Carrying,  for  example,  the  dried 
human  breath  into  a  tube  four  feet  long,  the  absorption 
there  effected  by  the  carbonic  acid  of  the  breath  amounts 
to  50  per  cent  of  the  entire  radiation.  Eadiant  heat  may 
indeed  be  employed  as  a  means  of  determining  practically 
the  amount  of  carbonic  acid  expired  from  the  lungs.  My 
late  assistant,  Mr.  Barrett,  while  under  my  direction,  made 
this  determination.  The  absorption  produced  by  the  breath 
freed  from  its  moisture,  but  retaining  its  carbonic  acid,  was 
first  determined.  Carbonic  acid,  artificially  prepared,  was 
then  mixed  with  dry  air  in  such  proportions  that  the  ac- 
tion of  the  mixture  upon  the  rays  of  heat  was  the  same  as 
that  of  the  dried  breath.  The  percentage  of  the  former 
being  known,  immediately  gave  that  of  the  latter.  The 
same  breath,  analyzed  chemically  by  Dr.  Frankland,  and 
physically  by  Mr,  Barrett,  gave  the  following  results: 

Percentage  of  Carbonic  Acid  in  the  Human  Breath 

Chemical  analysis  Pt  ycdca)  analyBiB 

4-66 *      .        .         .         .         4-66 

5-33        .         .         .         , 6-22 

BOIBKOB 4 


74  FRAGMENTS   OF  SCIENCE 

It  is  thus  proved  that  in  the  quantity  of  ethereal  mo- 
tion which  it  is  competent  to  take  up  we  have  a  practical 
measure  of  the  carbonic  acid  of  the  breath,  and  hence  of 
the  combustion  going  on  in  the  human  lungs. 

Still  this  question  of  period,  though  of  the  utmost  im- 
portance, is  not  competent  to  account  for  the  whole  of  the 
ol3served  facts.  The  ether,  as  far  as  we  know,  accepts 
vibrations  of  all  periods  with  the  same  readiness.  To  it 
the  oscillations  of  an  atom  of  free  oxygen  are  just  as  ac- 
ceptable as  those  of  the  atoms  in  a  molecule  of  olefiant 
gas;  that  the  vibrating  oxygen  then  stands  so  far  below 
the  olefiant  gas  in  radiant  power  must  be  referred  not  to 
period,  but  to  some  other  peculiarity.  The  atomic  group 
which  constitutes  the  molecule  of  olefiant  gas  produces 
many  thousand  times  the  disturbance  caused  by  the  oxy- 
gen, it  may  be  because  the  group  is  able  to  lay  a  vastly 
more  powerful  hold  upon  the  ether  than  the  single  atoms 
can.  Another,  and  probably  very  potent  cause  of  the  dif- 
ference may  be,  that  the  vibrations,  being  those  of  the  con- 
stituent atoms  of  the  molecule,*  are  generated  in  highly 
condensed  ether,  which  acts  like  condensed  air  upon  sound. 
But,  whatever  may  be  the  fate  of  these  attempts  to  visual- 
ize the  physics  of  the  process,  it  will  still  remain  true  that 
to  account  for  the  phenomena  of  radiation  and  absorption 
we  must  take  into  consideration  the  shape,  size,  and  con- 
dition of  the  ether  within  the  molecules,  by  which  the  ex- 
ternal ether  is  disturbed. 


'  See  "Physical  Considerations,"  Art.  iv.  p.  102. 


RADIATION  76 


16.  /Summary  and  Conclusion 

Let  us  now  cast  a  momentary  glance  over  the  gronnd 
that  we  have  left  behind.  The  general  nature  of  light  and 
heat  was  first  briefly  described:  the  compounding  of  mat- 
ter from  elementary  atoms,  and  the  influence  of  the  act  of 
combination  on  radiation  and  absorption,  were  considered 
and  experimentally  illustrated.  Through  the  transparent 
elementary  gases  radiant  heat  was  found  to  pass  as  through 
a  vacuum,  while  many  of  the  compound  gases  presented 
almost  impassable  obstacles  to  the  calorific  waves.  This 
deportment  of  the  simple  gases  directed  our  attention  to 
other  elementary  bodies,  the  examination  of  which  led  to 
the  discovery  that  the  element  iodine,  dissolved  in  bisid- 
phide  of  carbon,  possesses  the  power  of  detaching,  with 
extraordinary  sharpness,  the  light  of  the  spectrum  from  its 
heat,  intercepting  all  luminous  rays  up  to  the  extreme  red, 
and  permitting  the  calorific  rays  beyond  the  red  to  pass 
freely  through  it.  This  substance  was  then  employed  to 
filter  the  beams  of  the  electric  light,  and  to  form  foci  of 
invisible  rays  so  intense  as  to  produce  almost  all  the  effects 
obtainable  in  an  ordinary  fire.  Combustible  bodies  were 
burned,  and  refractory  ones  were  raised  to  a  white  heat,  by 
the  concentrated  invisible  rays.  Thus,  by  exalting  their 
refrangibility,  the  invisible  rays  of  the  electric  light  were 
rendered  visible,  and  all  the  colors  of  the  solar  spectrum 
were  extracted  from  utter  darkness.  The  extreme  richness 
of  the  electric  light  in  invisible  rays  of  low  refrangibility 
was  demonstrated,  one-eighth  only  of  its  radiation  consist- 
ing of  luminous  rays.  The  deadness  of  the  optic  nerve  to 
those  invisible  rays  was  proved,  and  experiments  were  then 


t6  FRAGMENTS   OF  SCIENCE 

added  to  show  tliat  the  bright  and  the  dark  rays  of  a  solid 
body,  raised  gradually  to  incandescence,  are  strengthened 
together;  intense  dark  heat  being  an  invariable  accompani- 
ment of  intense  white  heat.  A  sun  could  not  be  formed, 
or  a  meteorite  rendered  luminous,  on  any  other  condition. 
The  light-giving  rays  constituting  only  a  small  fraction  of 
the  total  radiation,  their  unspeakable  importance  to  us  is 
due  to  the  fact  that  their  periods  are  attuned  to  the  special 
requirements  of  the  eye. 

Among  the  vapors  of  volatile  liquids  vast  differences 
were  also  found  to  exist,  as  regards  their  powers  of  ab- 
sorption. "We  followed  various  molecules  from  a  state  of 
liquid  to  a  state  of  gas,  and  found,  in  both  states  of  aggre- 
gation, the  power  of  the  individual  molecules  equally  as- 
serted. The  position  of  a  vapor  as  an  absorber  of  radiant 
heat  was  shown  to  be  determined  by  that  of  the  liquid 
from  which  it  is  derived.  Eeversing  our  conceptions,  and 
regarding  the  molecules  of  gases  and  vapors  not  as  the  re- 
cipients, but  as  the  originators  of  wave -motion;  not  as  ab- 
sorbers, but  as  radiators;  it  was  proved  that  the  powers 
of  absorption  and  radiation  went  hand  in  hand,  the  self 
same  chemical  act  which  rendered  a  body  competent  to  in- 
tercept the  waves  of  ether  rendering  it  competent,  in  the 
same  degree,  to  generate  them.  Perfumes  were  next  sub- 
jected to  examination,  and,  notwithstanding  their  extraor- 
dinary tenuity,  they  were  found  vastly  superior,  in  point 
of  absorptive  power,  to  the  body  of  the  air  in  which  they 
were  diffused.  We  were  led  thus  slowly  up  to  the  exami- 
nation of  the  most  widely  diffused  and  most  important  of 
all  vapors — ^the  aqueous  vapor  of  our  atmosphere,  and  we 
found  in  it  a  potent  absorber  of  the  purely  calorific  rays. 
The  power  of  this  substance  to  influence  climate,  and  its 


RADIATION  77 

general  influence  on  the  temperature  of  the  earth,  were 
then  briefly  dwelt  upon.  A  cobweb  spread  above  a  blos- 
som is  sufficient  to  protect  it  from  nightly  chill;  and  thus 
the  aqueous  vapor  of  our  air,  attenuated  as  it  is,  checks 
the  drain  of  terrestrial  heat,  and  saves  the  surface  of  our 
planet  from  the  refrigeration  which  would  assuredly  ac- 
crue were  no  such  substance  interposed  between  it  and  the 
voids  of  space.  We  considered  the  influence  of  vibrating 
period,  and  molecular  form,  on  absorption  and  radiation, 
and  finally  deduced,  from  its  action  upon  radiant  heat, 
the  exact  amount  of  carbonic  acid  expired  by  the  human 
lungs. 

Thus,  in  brief  outline,  were  placed  before  you  some  of 
the  results  of  recent  inquiries  in  the  domain  of  Radiation, 
and  my  aim  throughout  has  been  to  raise  in  your  mind« 
distinct  physical  images  of  the  various  processes  involved 
in  our  researches.  It  is  thought  by  some  that  natural  sci- 
ence has  a  deadening  influence  on  the  imagination,  and  a 
doubt  might  fairly  be  raised  as  to  the  value  of  any  study 
which  would  necessarily  have  this  effect.  But  the  experi- 
ence of  the  last  hour  must,  I  think,  have  convinced  you 
that  the  study  of  natural  science  goes  hand  in  hand  with 
the  culture  of  the  imagination.  Throughout  the  greater 
part  of  this  discourse  we  have  been  sustained  by  this  fac- 
ulty. We  have  been  picturing  atoms,  and  molecules,  and 
vibrations,  and  waves,  which  eye  has  never  seen  nor  ear 
heard,  and  which  can  only  be  discerned  by  the  exercise  of 
imagination.  This,  in  fact,  is  the  faculty  which  enables  us 
to  transcend  the  boundaries  of  sense,  and  connect  the  phe- 
nomena of  our  visible  world  with  those  of  an  invisible 
one.  Without  imagination  we  never  could  have  risen  to 
the  conceptions  which  have  occupied  us  here  to-day;  and 


f8  FRAGMENTS   OF  SCIENCE 

in  proportion  to  jour  power  of  exercising  this  faculty 
aright,  and  of  associating  definite  mental  images  with  the 
terms  employed,  will  be  the  pleasure  and  the  profit  which 
you  will  derive  from  this  lecture.  The  outward  facts  of 
nature  are  insufficient  to  satisfy  the  mind.  We  cannot  be 
content  with  knowing  that  the  light  and  heat  of  the  sun 
illuminate  and  warm  the  world.  We  are  led  irresistibly  to 
inquire,  '*What  is  light,  and  what  is  heat?"  and  this 
question  leads  us  at  once  out  of  the  region  of  sense  into 
that  of  imagination.' 

Thus  pondering,  and  questioning,  and  striving  to  sup- 
plement that  which  is  felt  and  seen,  but  which  is  incom- 
plete, by  something  unfelt  and  unseen  which  is  necessary 
to  its  completeness,  men  of  genius  have  in  part  discerned, 
aot  only  the  nature  of  light  and  heat,  but  also,  through 
them,  the  general  relationship  of  natural  phenomena.  The 
working  power  of  Nature  consists  of  actual  or  potential 
motion,  of  which  all  its  phenomena  are  but  special  forms. 
This  motion  manifests  itself  in  tangible  and  in  intangible 
matter,  being  incessantly  transferred  from  one  to  the  other, 
and  incessantly  transformed  by  the  change.  It  is  as  real 
in  the  waves  of  the  ether  as  in  the  waves  of  the  sea;  the 
latter — derived  as  they  are  from  winds,  which  in  their  turn 
are  derived  from  the  sun — are,  indeed,  nothing  more  than 
the  heaped -up  motion  of  the  ether  waves.  It  is  the  calo- 
rific waves  emitted  by  the  sun  which  heat  our  air,  produce 
our  winds,  and  hence  agitate  our  ocean.  And  whether 
they  break  in  foam  upon  the  shore,  or  rub  silently  against 
the  ocean's  bed,  or  subside  by  the  mutual  friction  of  their 


*  This  lino  of  thought  was  pursued  further  five  years  subsequently.     See 
"Scientific  Use  of  the  Imagination"  in  Vol,  IL 


RADTATION  7d 

own  parts,  the  sea  waves,  which  cannot  subside  without 
producing  heat,  finally  resolve  themselves  into  waves  of 
ether,  thus  regenerating  the  motion  from  which  their  tem- 
porary existence  was  derived.  This  connection  is  typical. 
Nature  is  not  an  aggregate  of  independent  parts,  but  an 
organic  whole.  If  you  open  a  piano  and  sing  into  it,  a 
certain  string  will  respond.  Change  the  pitch  of  your 
voice;  the  first  string  ceases  to  vibrate,  but  another  re- 
plies. Change  again  the  pitch;  the  first  two  strings  are 
silent,  while  another  resounds.  Thus  is  sentient  man  acted 
on  by  Nature,  the  optic,  the  auditory,  and  other  nerves  of 
the  human  body  being  so  many  strings  differently  tuned, 
and  responsive  to  different  forms  of  the  universal  power. 


Ill 


ON  RADIANT  HEAT  IN  RELATION  TO  THE  COLOR  AND 
CHEMICAL  CONSTITUTION  OF  BODIES* 

ONE  of  the  most  important  functions  of  physical 
science,  considered  as  a  discipline  of  the  mind,  is 
to  enable  us  by  means  of  the  sensible  processes 
of  Nature  to  apprehend  the  insensible.  The  sensible  proc- 
esses give  direction  to  the  line  of  thought;  but  this  once 
given,  the  length  of  the  line  is  not  limited  by  the  bounda- 
ries of  the  senses.  Indeed,  the  domain  of  the  senses,  in 
Nature,  is  almost  infinitely  small  in  comparison  with  the 
vast  region  accessible  to  thought  which  lies  beyond  them. 
From  a  few  observations  of  a  comet,  when  it  comes  within 
the  range  of  his  telescope,  an  astronomer  can  calculate  its 
path  in  regions  which  no  telescope  can  reach:  and  in  like 
manner,  by  means  of  data  furnished  in  the  narrow  world 
of  the  senses,  we  make  ourselves  at  home  in  other  and 
wider  worlds,  which  are  traversed  by  the  intellect  alone. 
From  the  earliest  ages  the  questions,  *'What  is  light?'* 
and  **What  is  heat?"  have  occurred  to  the  minds  of  men; 
but  these  questions  never  would  have  been  answered  had 
they  not  been  preceded  by  the  question,  *' What  is  sound  ?'* 
Amid  the  grosser  phenomena  of  acoustics  the  mind  was 
first  disciplined,  conceptions  being  thus  obtained  from  direct 


1  A  discourse  delivered  in  the  Rojal  Institution  of  Great  Britain,  January 
19,  1866. 

(80) 


RADIANT  HEAT  AND   ITS   RELATIONS  81 

observation,  which  were  afterward  applied  to  phenomena  of  a 
character  far  too  subtle  to  be  observed  directly.  Sound  we 
know  to  be  due  to  vibratory  motion.  A  vibrating  tuning- 
fork,  for  example,  molds  the  air  around  it  into  undula- 
tions or  waves,  which  speed  away  on  all  sides  with  a  cer- 
tain measured  velocity,  impinge  upon  the  drum  of  the  ear, 
shake  the  auditory  nerve,  and  awake  in  the  brain  the  sen- 
sation of  sound.  When  sufficiently  near  a  sounding  body 
we  can  feel  the  vibrations  of  the  air.  A  deaf  man,  for  ex- 
ample, plunging  his  hand  into  a  bell  when  it  is  sounded, 
feels  through  the  common  nerves  of  his  body  those  tremors 
which,  when  imparted  to  the  nerves  of  healthy  ears,  are 
translated  into  sound.  There  are  various  ways  of  render- 
ing those  sonorous  vibrations  not  only  tangible,  but  visi- 
ble; and  it  was  not  until  numberless  experiments  of  this 
kind  had  been  executed  that  the  scientific  investigator 
abandoned  himself  wholly,  and  without  a  shadow  of  mis- 
giving, to  the  conviction  that  what  is  sound  within  us  is, 
outside  of  us,  a  motion  of  the  air. 

But  once  having  established  this  fact— once  having 
proved,  beyond  all  doubt,  that  the  sensation  of  sound 
is  produced  by  an  agitation  of  the  auditory  nerve — the 
thought  soon  suggested  itself  that  light  might  be  due  to 
an  agitation  of  the  optic  nerve.  This  was  a  great  step  in 
advance  of  that  ancient  notion  which  regarded  light  as 
something  emitted  by  the  eye,  and  not  as  anything  im- 
parted to  it.  But  if  light  be  produced  by  an  agitation  of 
the  retina,  what  is  it  that  produces  the  agitation?  New- 
ton, you  know,  supposed  minute  particles  to  be  shot 
through  the  humors  of  the  eye  against  the  retina,  which 
he  supposed  to  hang  like  a  target  at  the  back  of  the  eye. 
The  impact  of  these  particles  against  the  target,  Newton 


62  FRAGMENTS   OF  SCIENCE 

believed  to  be  the  cause  of  light.  But  Newton's  notion 
has  not  held  its  ground,  being  entirely  driven  from  the 
field  by  the  more  wonderful  and  far  more  philosophical 
notion  that  light,  like  so\ind,  is  a  product  of  wave- 
motion. 

The  domain  in  which  this  motion  of  light  is  carried  on 
lies  entirely  beyond  the  reach  of  our  senses.  The  waves 
of  light  require  a  medium  for  their  formation  and  propa- 
gation: but  we  cannot  see,  or  feel,  or  taste,  or  smell  this 
medium.  How,  then,  has  its  existence  been  established? 
By  showing  that,  by  the  assumption  of  this  wonderful  in- 
tangible ether,  all  the  phenomena  of  optics  are  accounted 
for,  with  a  fulness,  and  clearness,  and  conclusiveness, 
which  leave  no  desire  of  the  intellect  unsatisfied.  When 
the  law  of  gravitation  first  suggested  itself  to  the  mind  of 
Newton,  what  did  he  do?  He  set  himself  to  examine 
whether  it  accoimted  for  all  the  facts.  He  determined  the 
courses  of  the  planets;  he  calculated  the  rapidity  of  the 
moon's  fall  toward  the  earth;  he  considered  the  precession 
of  the  equinoxes,  the  ebb  and  flow  of  the  tides,  and  found 
all  explained  by  the  law  of  gravitation.  He  therefore  re- 
garded this  law  as  established,  and  the  verdict  of  science 
subsequently  confirmed  his  conclusion.  On  similar,  and,  if 
possible,  on  stronger  grounds,  we  found  our  belief  in  the 
existence  of  the  universal  ether.  It  explains  facts  far  more 
various  and  complicated  than  those  on  which  Newton  based 
his  law.  If  a  single  phenomenon  could  be  pointed  out 
which  the  ether  is  proved  incompetent  to  explain,  we 
should  have  to  give  it  up;  but  no  such  phenomenon  has 
ever  been  pointed  out.  It  is,  therefore,  at  least  as  certain 
that  space  is  filled  with  a  medium,  by  means  of  which  suns 
and  stars  difEuse  their  radiant  power,  as  that  it  is  traversed 


RADIANT  HEAT  AND   ITS   RELATIONS  88 

by  that  force  which  holds  in  its  grasp,  not  only  our  plan- 
etary system,  but  the  immeasurable  heavens  themselves. 

There  is  no  more  wonderful  instance  than  this  of  the 
production  of  a  line  of  thought,  from  the  world  of  the 
senses  into  the  region  of  pure  imagination.  I  mean  by 
imagination  here,  not  that  play  of  fancy  which  can  give 
to  airy  nothings  a  local  habitation  and  a  name,  but  that 
power  which  enables  the  mind  to  conceive  realities  which 
lie  beyond  the  range  of  the  senses — to  present  to  itseK  dis- 
tinct images  of  processes  which,  though  mighty  in  the  ag- 
gregate beyond  all  conception,  are  so  minute  individually 
as  to  elude  all  observation.  It  is  the  waves  of  air  excited 
by  a  tuning-fork  which  render  its  vibrations  audible.  It 
is  the  waves  of  ether  sent  forth  from  those  lamps  overhead 
which  render  them  luminous  to  us;  but  so  minute  are  these 
waves,  that  it  would  take  from  30,000  to  60,000  of  them, 
placed  end  to  end,  to  cover  a  single  inch.  Their  number, 
however,  compensates  for  their  minuteness.  Trillions  of 
them  have  entered  your  eyes,  and  hit  the  retina  at  the 
backs  of  your  eyes,  in  the  time  consumed  in  the  utterance 
of  the  shortest  sentence  of  this  discourse.  This  is  the 
steadfast  result  of  modern  research;  but  we  never  could 
have  reached  it  without  previous  discipline.  We  never 
could  have  measured  the  waves  of  light,  nor  even  imag- 
ined them  to  exist,  had  we  not  previously  exercised  our- 
selves among  the  waves  of  sound.  Sound  and  light  are 
now  mutually  helpful,  the  conceptions  of  each  being  ex- 
panded, strengthened,  and  defined  by  the  conceptions  of 
the  other. 

The  ether  which  conveys  the  pulses  of  light  and  heat 
not  only  fills  celestial  space,  swathing  suns,  and  planets, 
and  moons,  but  it  also  encircles  the  atoms  of  which  these 


Si  FRAGMENTS   OF  SCIENCE 

bodies  are  composed.  It  is  the  motion  of  these  atoms,  and 
not  that  of  any  sensible  parts  of  bodies,  that  the  ether  con- 
veys. This  motion  is  the  objective  cause  of  what,  in  our 
sensations,  are  light  and  heat.  An  atom,  then,  sending  its 
pulses  through  the  ether,  resembles  a  tuning-fork  sending 
its  pulses  through  the  air.  Let  us  look  for  a  moment  at 
this  thrilling  medium,  and  briefly  consider  its  relation  to 
the  bodies  whose  vibrations  it  conveys.  Different  bodies, 
when  heated  to  the  same  temperature,  possess  very  differ- 
ent powers  of  agitating  the  ether:  some  are  good  radiators, 
others  are  bad  radiators;  which  means  that  some  are  so 
constituted  as  to  communicate  their  atomic  motion  freely 
to  the  ether,  producing  therein  powerful  undulations; 
while  the  atoms  of  others  are  unable  thus  to  communicate 
their  motions,  but  glide  through  the  medium  without  ma- 
terially disturbing  its  repose.  Recent  experiments  have 
proved  that  elementary  bodies,  except  under  certain  anom- 
alous conditions,  belong  to  the  class  of  bad  radiators.  An 
atom,  vibrating  in  the  ether,  resembles  a  naked  tuning- 
fork  vibrating  in  the  air.  The  amount  of  motion  commu- 
nicated to  the  air  by  the  thin  prongs  is  too  small  to  evoke 
at  any  distance  the  sensation  of  sound.  But  if  we  permit 
the  atoms  to  combine  chemically  and  form  molecules,  the 
result,  in  many  cases,  is  an  enormous  change  in  the  power 
of  radiation.  The  amount  of  ethereal  disturbance,  pro- 
duced by  the  combined  atoms  of  a  body,  may  be  many 
thousand  times  that  produced  by  the  same  atoms  when 
uncombined. 

The  pitch  of  a  musical  note  depends  upon  the  rapidity 
of  its  vibrations,  or,  in  other  words,  on  the  length  of  its 
waves.  Now,  the  pitch  of  a  note  answers  to  the  color  of 
light.    Taking  a  slice  of  white  light  from  the  sun,  or  from 


RADIANT  HEAT  AND  ITS  RELATIONS  85 

an  electric  lamp,  and  causing  the  light  to  pass  through  an 
arrangement  of  prisms,  it  is  decomposed.  We  have  the 
effect  obtained  by  Newton,  who  first  unrolled  the  solar 
beam  into  the  splendors  of  the  solar  spectrum.  At  one 
end  of  this  spectrum  we  have  red  light,  at  the  other,  vio- 
let; and  between  those  extremes  lie  the  other  prismatic 
colors.  As  we  advance  along  the  spectrum  from  the  red 
to  the  violet,  the  pitch  of  the  light — ^if  I  may  use  the  ex- 
pression— heightens,  the  sensation  of  violet  being  produced 
by  a  more  rapid  succession  of  impulses  than  that  which 
produces  the  impression  of  red.  The  vibrations  of  the 
violet  are  about  twice  as  rapid  as  those  of  the  red;  in 
other  words,  the  range  of  the  visible  spectrum  is  about 
an  octave. 

There  is  no  solution  of  continuity  in  this  spectrum; 
one  color  changes  into  another  by  insensible  gradations. 
It  is  as  if  an  infinite  number  of  tuning-forks,  of  gradually 
augmenting  pitch,  were  vibrating  at  the  same  time.  But 
turn?ng  to  another  spectrum — ^that,  namely,  obtained  from 
the  incandescent  vapor  of  silver — ^you  observe  that  it  con- 
sists of  two  narrow  and  intensely  luminous  green  bands. 
Here  it  is  as  if  two  forks  only,  of  slightly  different  pitch, 
were  vibrating.  The  length  of  the  waves  which  produce 
this  first  band  is  such  that  47,460  of  them,  placed  end  to 
end,  would  fill  an  inch.  The  waves  which  produce  the 
second  band  are  a  little  shorter;  it  would  take  of  these 
47,920  to  fill  an  inch.  In  the  case  of  the  first  band,  the 
number  of  impulses  imparted,  in  one  second,  to  every  eye 
which  sees  it,  is  577  millions  of  millions;  while  the  num- 
ber of  impulseij  imparted,  in  the  same  time,  by  the  second 
band,  is  600  millions  of  millions.  We  may  project  upon 
a  white  screen  the  beautiful  stream  of  green  light  from 


«6  FRAGMENTS   OF  SCIENCE 

which  these  bands  were  derived.  This  luminous  stream  is 
the  incandescent  vapor  of  silver.  The  rates  of  vibration 
of  the  atoms  of  that  vapor  are  as  rigidly  fixed  as  those  of 
two  tuning-forks;  and  to  whatever  height  the  temperature 
of  the  vapor  may  be  raised,  the  rapidity  of  its  vibrations, 
and  consequently  its  color,  which  wholly  depends  upon 
that  rapidity,  remain  unchanged. 

The  vapor  of  water,  as  well  as  the  vapor  of  silver,  has 
its  definite  periods  of  vibration,  and  these  are  such  as  to 
disqualify  the  vapor,  when  acting  freely  as  such,  from  be- 
ing raised  to  a  white  heat.  The  oxy hydrogen  flame,  for 
example,  consists  of  hot  aqueous  vapor.  It  is  scarcely  vis- 
ible in  the  air  of  this  room,  and  it  would  be  still  less  visi- 
ble if  we  could  bum  the  gas  in  a  clean  atmosphere.  But 
the  atmosphere,  even  at  the  summit  of  Mont  Blanc,  is 
dirty;  in  London  it  is  more  than  dirty;  and  the  burning 
dirt  gives  to  this  flame  the  greater  portion  of  its  present 
light.  But  the  heat  of  the  flame  is  enormous.  Cast  iron 
fuses  at  a  temperature  of  2,000°  Fahr. ;  while  the  tempera- 
ture of  the  oxy  hydrogen  flame  is  6,000°  Fahr.  A  piece  of 
platinum  is  heated  to  vivid  redness,  at  a  distance  of  two 
inches  beyond  the  visible  termination  of  the  flame.  The 
vapor  which  produces  incandescence  is  here  absolutely 
dark.  In  the  flame  itself  the  platinum  is  raised  to  daz- 
zling whiteness,  and  is  even  pierced  by  the  flame.  When 
this  flame  impinges  on  a  piece  of  lime  we  have  the  daz- 
zling Drummond  light.  But  the  light  is  here  due  to  the 
fact  that  when  it  impinges  upon  the  solid  body,  the  vibra- 
tions excited  in  that  body  by  the  flame  are  of  periods 
different  from  its  own. 

Thus  far  we  have  fixed  our  attention  on  atoms  and 
molecules  in  a  state  of   vibration,  and  surrounded  by  a 


RADIANT  HEAT  AND   ITS   RELATIONS  87 

medium  which  accepts  their  vibrations,  and  transmits  them 
through  space.  But  suppose  the  waves  generated  by  one 
system  of  molecules  to  impinge  upon  another  system,  how 
will  the  waves  be  affected  ?  Will  they  be  stopped,  or  will 
they  be  permitted  to  pass?  Will  they  transfer  their  mo- 
tion to  the  molecules  on  which  they  impinge,  or  will  they 
glide  round  the  molecules,  through  the  intermolecular 
spaces,   and  thus  escape? 

The  answer  to  this  question  depends  upon  a  condition 
which  may  be  beautifully  exemplified  by  an  experiment 
on  sound.  These  two  tuning-forks  are  tuned  absolutely 
alike.  They  vibrate  with  the  same  rapidity,  and,  mounted 
thus  upon  their  resonant  cases,  you  hear  them  loudly 
sounding  the  same  musical  note.  Stopping  one  of  the 
forks,  I  throw  the  other  into  strong  vibration,  and  bring 
that  other  near  the  silent  fork,  but  not  into  contact  with 
it.  Allowing  them  to  continue  in  this  position  for  four 
or  five  seconds,  and  then  stopping  the  vibrating  fork,  the 
sound  does  not  cease.  The  second  fork  has  taken  up  the 
vibrations  of  its  neighbor,  and  is  now  sounding  in  its  turn. 
Dismounting  one  of  the  forks,  and  permitting  the  other  to 
remain  upon  its  stand,  I  throw  the  dismounted  fork  into 
stroixg  vibration.  You  cannot  hear  it  sound.  Detached 
from  its  case,  the  amount  of  motion  which  it  can  commu- 
nicate to  the  air  is  too  small  to  be  sensible  at  any  dis- 
tance. When  the  dismounted  fork  is  brought  close  to  the 
mounted  one,  but  not  into  actual  contact  with  it,  out  of 
the  silence  rises  a  mellow  sound.  Whence  comes  it? 
From  the  vibrations  which  have  been  transferred  from 
the  dismounted  fork  to  the  mounted  one. 

That  the  motion  should  thus  transfer  itself  through  the 
air  it  is  necessary  that  the  two  forks  should  be  in  perfect 


68  FRAGMENTS   OF  SCIENCE 

unison.  If  a  morsel  of  wax,  not  larger  than  a  pea,  be 
placed  on  one  of  the  forks,  it  is  rendered  thereby  power- 
less to  affect,  or  to  be  affected  by,  the  other.  It  is  easy 
to  understand  this  experiment.  The  pulses  of  the  one  fork 
can  affect  the  other,  because  they  are  perfectly  timed.  A 
single  pulse  causes  the  prong  of  the  silent  fork  to  vibrate 
through  an  infinitesimal  space.  But  just  as  it  has  com- 
pleted this  small  vibration,  another  pulse  is  ready  to  strike 
it.  Thus,  the  impulses  add  themselves  together.  In  the 
five  seconds  during  which  the  forks  were  held  near  each 
other,  the  vibrating  fork  sent  1,280  waves  against  its  neigh- 
bor, and  those  1,280  shocks,  all  delivered  at  the  proper 
moment,  all,  as  I  have  said,  perfectly  timed,  have  given 
such  strength  to  the  vibrations  of  the  mounted  fork  as  to 
render  them  audible  to  all. 

Another  curious  illustration  of  the  influence  of  syn- 
chronism on  musical  vibrations,  is  this:  Three  small  gas- 
flames  are  inserted  into  three  glass  tubes  of  different 
lengths.  Each  of  these  flames  can  be  caused  to  emit  a 
musical  note,  the  pitch  of  which  is  determined  by  the 
length  of  the  tube  surrounding  the  flame.  The  shorter 
the  tube  the  higher  is  the  pitch.  The  flames  are  now  si- 
lent within  their  respective  tubes,  but  each  of  them  can 
be  caused  to  respond  to  a  proper  note  sounded  anywhere  in 
this  room.  With  an  instrument  called  a  syren,  a  power- 
ful musical  note,  of  gradually  increasing  pitch,  can  be  pro- 
duced. Beginning  with  a  low  note,  and  ascending  grad- 
ually to  a  higher  one,  we  finally  attain  the  pitch  of  the 
flame  in  the  longest  tube.  The  moment  it  is  reached,  the 
flame  bursts  into  song.  The  other  flames  are  still  silent 
within  their  tubes.  But,  by  urging  the  instrument  on  to 
higher  notes,  the  second  flame  is  started,    and  the  third 


RADIANT  HEAT  AND   ITS   RELATIONS  89 

alone  remains.  A  still  higher  note  starts  it  also.  Thus, 
as  the  sound  of  the  syren  rises  gradually  in  pitch,  it 
awakens  every  flame  in  passing,  by  striking  it  with  a 
series  of  waves  whose  periods  of  recurrence  are  similar 
to  its  own. 

Now  the  wave-motion  from  the  syren  is  in  part  taken 
up  by  the  flame  which  synchronizes  with  the  waves;  and 
were  these  waves  to  impinge  upon  a  multitude  of  flames, 
instead  of  upon  one  flame  only,  the  Iransference  might  be 
so  great  as  to  absorb  the  whole  of  the  original  wave-mo- 
tion. Let  ua  apply  these  facts  to  radiant  heat.  This  blue 
flame  is  the  flame  of  carbonic  oxide;  this  transparent  gas 
is  carbonic  acid  gas.  In  the  blue  flame  we  have  carbonic 
acid  intensely  heated,  or,  in  other  words,  in  a  state  of  in- 
tense vibration.  It  thus  resembles  the  sounding  fork,  while 
this  cold  carbonic  acid  resembles  the  silent  one.  What  is 
the  consequence?  Through  the  synchronism  of  the  hot 
and  cold  gas,  the  waves  emitted  by  the  former  are  inter- 
cepted by  the  latter,  the  transiiission  of  the  radiant  heat 
being  thus  prevented.  The  cold  gas  is  intensely  opaque 
to  the  radiation  from  this  particular  flame,  though  highly 
transparent  to  heat  of  every  other  kind.  We  are  here 
manifestly  dealing  with  that  great  principle  which  lies  at 
the  basis  of  spectrum  analysis,  and  which  has  enabled  sci- 
entific men  to  determine  the  substances  of  which  the  sun, 
the  stars,  and  even  the  nebula,  are  composed;  the  princi- 
ple, namely,  that  a  body  which  is  competent  to  emit  any 
ray,  whether  of  heat  or  light,  is  competent  in  the  same  de- 
gree to  absorb  that  ray.  The  absorption  depends  on  the 
synchronism  existing  between  the  vibrations  of  the  atoms 
from  which  the  rays,  or  more  correctly  the  waves,  issue, 
and  those  of  the  atoms  on  which  they  impinge. 


90  FRAGMENTS   OF  SCIENCE 

To  its  almost  total  incompetence  to  emit  white  light, 
aqueous  vapor  adds  a  similar  incompetence  to  absorb  white 
light.  It  cannot,  for  example,  absorb  the  luminous  rays 
of  the  sun,  though  it  can  absorb  the  non-luminous  rays  of 
the  earth.  This  incompetence  of  the  vapor  to  absorb  lu- 
minous rays  is  shared  by  water  and  ice — in  fact,  by  all 
really  transparent  substances.  Their  transparency  is  due 
to  their  inability  to  absorb  luminous  rays.  The  molecules 
of  such  substances  are  in  dissonance  with  the  luminous 
waves;  and  hence  such  waves  pass  through  transparent 
bodies  without  disturbing  the  molecular  rest.  A  purely 
luminous  beam,  however  intense  may  be  its  heat,  is  sen- 
sibly incompetent  to  melt  ice.  We  can,  for  example,  con- 
verge a  powerful  luminous  beam  upon  a  surface  covered 
with  hoar  frost,  without  melting  a  single  spicula  of  the 
crystals.  How  then,  it  may  be  asked,  are  the  snows  of 
the  Alps  swept  away  by  the  sunshine  of  summer?  I  an- 
swer, they  are  not  swept  away  by  sunshine  at  all,  but  by 
rays  which  have  no  sunshine  whatever  in  them.  The  lu- 
minous rays  of  the  sun  fall  upon  the  snow-fields  and  are 
flashed  in  echoes  from  crystal  to  crystal,  but  they  find 
next  to  no  lodgment  within  the  crystals.  They  are  hardly 
at  all  absorbed,  and  hence  they  cannot  produce  fusion. 
But  a  body  of  powerful  dark  rays  is  emitted  by  the  sun; 
and  it  is  these  that  cause  the  glaciers  to  shrink  and  the 
snows  to  disappear;  it  is  they  that  fill  the  banks  of  the 
Arve  and  Arveyron,  and  liberate  from  their  frozen  captiv- 
ity the  Ehone  and  the  Rhine. 

Placing  a  concave  silvered  mirror  behind  the  electric 
light  its  rays  are  converged  to  a  focus  of  dazzling  bril- 
liancy. Placing  in  the  path  of  the  rays,  between  the  light 
and  the  focus,  a  vessel  of  water,  and  introducing  at  the 


RADIANT  HEAT  AND   ITS   RELATIONS  91 

focus  a  piece  of  ice,  the  ice  is  not  melted  by  the  concen- 
trated beam.  Matches,  at  the  same  place,  are  ignited,  and 
wood  is  set  on  fire.  The  powerful  heat,  then,  of  this  lu- 
minous beam  is  incompetent  to  melt  the  ice.  On  with- 
drawing the  cell  of  water,  the  ice  immediately  liquefies, 
and  the  water  trickles  from  it  in  drops.  Eeintroducing  the 
cell  of  water,  the  fusion  is  arrested,  and  the  drops  cease 
to  fall.  The  transparent  water  of  the  cell  exerts  no  sen- 
sible absorption  on  the  luminous  rays,  still  it  withdraws 
something  from  the  beam,  which,  when  permitted  to  act,  is 
competent  to  melt  the  ice.  This  something  is  the  dark 
radiation  of  the  electric  light.  Again,  I  place  a  slab  of 
pure  ice  in  front  of  the  electric  lamp;  send  a  luminous 
beam  first  through  our  cell  of  water  and  then  through  the 
ice.  By  means  of  a  lens  an  image  of  the  slab  is  cast  upon 
a  white  screen.  The  beam,  sifted  by  the  water,  has  little 
pow6r  upon  the  ice.  But  observe  what  occurs  when  the 
water  is  removed;  we  have  here  a  star  and  there  a  star, 
each  star  resembling  a  flower  of  six  petals,  and  growing 
visibly  larger  before  our  eyes.  As  the  leaves  enlarge,  their 
edges  become  serrated,  but  there  is  no  deviation  from  the 
six-rayed  type.  We  have  here,  in  fact,  the  crystallization 
of  the  ice  reversed  by  the  invisible  rays  of  the  electric 
beam.  They  take  the  molecules  down  in  this  wonderful 
way,  and  reveal  to  us  the  exquisite  atomic  structure  of 
the  substance  with  which  Nature  every  winter  roofs  our 
ponds  and  lakes. 

Numberless  effects,  apparently  anomalous,  might  be  ad- 
duced in  illustration  of  the  action  of  these  lightless  rays. 
These  two  powders,  for  example,  are  both  white,  and  in- 
distinguishable from  each  other  by  the  eye.  The  luminous 
rays  of  the  sun  are  unabsorbed  by  both — ^from  such  rays 


92  FRAGMENTS   OF  SCIENCE 

these  powders  acquire  no  heat;  still  one  of  them,  sugar,  is 
heated  so  highly  by  the  concentrated  beam  of  the  electric 
lamp  that  it  first  smokes  and  then  violently  inflames,  while 
the  other  substance,  salt,  is  barely  warmed  at  the  focus. 
Placing  two  perfectly  transparent  liquids  in  test-tubes  at 
the  focus,  one  of  them  boils  in  a  couple  of  seconds,  while 
the  other,  in  a  similar  position,  is  hardly  warmed.  The 
boiling-point  of  the  first  liquid  is  78°  C,  which  is  speedily 
reached;  that  of  the  second  liquid  is  only  48°  C,  which  is 
never  reached  at  all.  These  anomalies  are  entirely  due  to 
the  unseen  element  which  mingles  with  the  luminous  rays 
of  the  electric  beam,  and  indeed  constitutes  90  per  cent  of 
its  calorific  power. 

A  substance,  as  many  of  you  know,  has  been  discov- 
ered, by  which  these  dark  rays  may  be  detached  from  the 
total  emission  of  the  electric  lamp.  This  ray-filter  is  a 
liquid,  black  as  pitch  to  the  luminous,  but  bright  as  a  dia- 
mond to  the  non-luminous,  radiation.  It  mercilessly  cuts 
off  the  former,  but  allows  the  latter  free  transmission. 
When  these  invisible  rays  are  brought  to  a  focus,  at  a 
distance  of  several  feet  from  the  electric  lamp,  the  dark 
rays  form  an  invisible  image  of  their  source.  By  proper 
means,  this  image  may  be  transformed  into  a  visible  one 
of  dazzling  brightness.  It  might,  moreover,  be  shown,  if 
time  permitted,  how,  out  of  those  perfectly  dark  rays, 
could  be  extracted,  by  a  process  of  transmutation,  all  the 
colors  of  the  solar  spectrum.  It  might  also  be  proved  that 
those  rays,  powerful  as  they  are,  and  sufficient  to  fuse 
m&ny  metals,  can  be  permitted  to  enter  the  eye,  and  to 
break  upon  the  retina,  without  producing  the  least  lumi- 
nous impression. 

The  dark  rays  being  thus  collected,  you  see  nothing  at 


RADIANT  HEAT  AND   ITS   RELATIONS  93 

their  place  of  convergence.  With  a  proper  thermometer 
it  could  be  proved  that  even  the  air  at  the  focus  is  just  as 
cold  as  the  surrounding  air.  And  mark  the  conclusion  to 
which  this  leads.  It  proves  the  ether  at  the  focus  to  be 
practically  detached  from  the  air — that  the  most  violent 
ethereal  motion  may  there  exist,  without  the  least  aerial 
motion.  But,  though  you  see  it  not,  there  is  sufficient  heat 
at  that  focus  to  set  London  on  fire.  The  heat  there  is 
competent  to  raise  iron  to  a  temperature  at  which  it  throws 
off  brilliant  scintillations.  It  can  heat  platinum  to  white- 
ness, and  almost  fuse  that  refractory  metal.  It  actually 
can  fuse  gold,  silver,  copper,  and  aluminium.  The  mo- 
ment, moreover,  that  wood  is  placed  at  the  focus  it  bursts 
into  a  blaze. 

It  has  been  already  affirmed  that,  whether  as  regards 
radiation  or  absorption,  the  elementary  atoms  possess  but 
little  power.  This  might  be  illustrated  by  a  long  array  of 
facts;  and  one  of  the  most  singular  of  these  is  furnished 
by  the  deportment  of  that  extremely  combustible  sub- 
stance, phosphorus,  when  placed  at  the  dark  focus.  It 
is  impossible  to  ignite  there  a  fragment  of  amorphous 
phosphorus.  But  ordinary  phosphorus  is  a  far  quicker 
combustible,  and  its  deportment  toward  radiant  heat  is 
still  more  impressive.  It  may  be  exposed  to  the  intense 
radiation  of  an  ordinary  fire  without  bursting  into  flame. 
It  may  also  be  exposed  for  twenty  or  thirty  seconds  at 
an  obscure  focus,  of  sufficient  power  to  raise  platinum  to 
a  red  heat,  without  ignition.  Notwithstanding  the  energy 
of  the  ethereal  waves  here  concentrated,  notwithstanding 
the  extremely  inflammable  character  of  the  elementary 
body  exposed  to  their  action,  the  atoms  of  that  body 
refuse  to  partake  of  the  motion  of  the  powerful  waves 


94  FRAGMENTS   OF  SCIENCE 

of  low  refrangibilitj,  and  consequently  cannot  be  affected 
by  their  heat. 

The  knowledge  we  now  possess  will  enable  us  to  ana- 
lyze with  profit  a  practical  question.  White  dresses  are 
worn  in  summer,  because  they  are  found  to  be  cooler  than 
dark  ones.  The  celebrated  Benjamin  Franklin  placed  bits 
of  cloth  of  various  colors  upon  snow,  exposed  them  to  di- 
rect sunshine,  and  found  that  they  sank  to  different  depths 
in  the  snow.  The  black  cloth  sank  deepest,  the  white  did 
not  sink  at  all.  Franklin  inferred  from  this  experiment 
that  black  bodies  are  the  best  absorbers,  and  white  ones 
the  worst  absorbers,  of  radiant  heat.  Let  us  test  the  gen- 
erality of  this  conclusion.  One  of  these  two  cards  is  coated 
with  a  very  dark  powder,  and  the  other  with  a  perfectly 
white  one.  I  place  the  powdered  surfaces  before  a  fire, 
and  leave  them  there  until  they  have  acquired  as  high  a 
temperature  as  they  can  attain  in  this  position.  Which  of 
the  cards  is  then  most  highly  heated  ?  It  requires  no  ther- 
mometer to  answer  this  question.  Simply  pressing  the 
back  of  the  card,  on  which  the  white  powder  is  strewn, 
against  the  cheek  or  forehead,  it  is  found  intolerably  hot. 
Placing  the  dark  card  in  the  same  position,  it  is  found 
cool.  The  white  powder  has  absorbed  far  more  heat  than 
the  dark  one.  This  simple  result  abolishes  a  hundred  con- 
clusions which  have  been  hastily  drawn  from  the  experi- 
ment of  Franklin.  Again,  here  are  suspended  two  deli- 
cate mercurial  thermometers  at  the  same  distance  from  a 
gas -flame.  The  bulb  of  one  of  them  is  covered  by  a  dark 
substance,  the  bulb  of  the  other  by  a  white  one.  Both 
bulbs  have  received  the  radiation  from  the  flame,  but  the 
white  bulb  has  absorbed  most,  and  its  mercury  stands 
much  higher  than  that  of  the  other  thermometer.     This 


RADIANT  HEAT  AND    ITS   RELATIONS  95 

experiment  might  be  varied  in  a  hundred  ways:  it  proves 
that  from  the  darkness  of  a  body  you  can  draw  no  certain 
conclusion  regarding  its  power  of  absorption. 

The  reason  of  this  simply  is,  that  color  gives  us  intelli- 
gence of  only  one  portion,  and  that  the  smallest  one,  of 
the  rays  impinging  on  the  colored  body.  Were  the  rays 
all  luminous,  we  might  with  certainty  infer  from  the  color 
of  a  body  its  power  of  absorption;  but  the  great  mass  of 
the  radiation  from  our  fire,  our  gas-flame,  and  even  from 
the  sun  itself,  consists  of  invisible  calorific  rays,  regarding 
which  color  teaches  us  nothing.  A  body  may  be  highly 
transparent  to  the  one  class  of  rays,  and  highly  opaque  to 
the  other.  Thus  the  white  powder,  which  has  shown  itself 
so  powerful  an  absorber,  has  been  specially  selected  on  ac- 
count of  its  extreme  perviousness  to  the  visible  rays,  and 
its  extreme  imperviousness  to  the  invisible  ones;  while  the 
dark  powder  was  chosen  on  account  of  its  extreme  trans- 
parency to  the  invisible,  and  its  extreme  opacity  to  the 
visible,  rays.  In  the  case  of  the  radiation  from  our  fire, 
about  98  per  cent  of  the  whole  emission  consists  of  invis- 
ible rays;  the  body,  therefore,  which  was  most  opaque  to 
these  triumphed  as  an  absorber,  though  that  body  was  a 
white  one. 

And  here  it  is  worth  while  to  consider  the  manner  in 
which  we  obtain  from  natural  facts  what  may  be  called 
their  intellectual  value.  Throughout  the  processes  of  Na- 
ture we  have  interdependence  and  harmony;  and  the  main 
value  of  physics,  considered  as  a  mental  discipline,  con- 
sists in  the  tracing  out  of  this  interdependence,  and  the 
demonstration  of  this  harmony.  The  outward  and  visible 
phenomena  are  the  counters  of  the  intellect;  and  our  sci- 
ence  would   not  be   worthy  of  its   name   and  fame  if  it 


96  FRAGMENTS   OF  SCIENCE 

halted  at  facts,  however  practically  useful,  and  neglected 
the  laws  which  accompany  and  rule  the  phenomena.  Let 
us  endeavor  then  to  extract  from  the  experiment  of  Frank- 
lin all  that  it  can  yield,  calling  to  our  aid  the  knowledge 
which  our  predecessors  have  already  stored.  Let  us  im- 
agine two  pieces  of  cloth  of  the  same  texture,  the  one  black 
and  the  other  white,  placed  upon  sunned  snow.  Fixing 
our  attention  on  the  white  piece,  let  us  inquire  whether 
there  is  any  reason  to  expect  that  it  will  sink  in  the  snow 
at  all.  There  is  knowledge  at  hand  which  enables  us  to 
reply  at  once  in  the  negative.  There  is,  on  the  contrary, 
reason  to  expect  that,  after  a  sufficient  exposure,  the  bit 
of  cloth  will  be  found  on  an  eminence  instead  of  in  a  hol- 
low; that,  instead  of  a  depression,  we  shall  have  a  relative 
elevation  of  the  bit  of  cloth.  For,  as  regards  the  laminoua 
rays  of  the  sun,  tlie  cloth  and  the  snow  are  alike  power- 
less; the  one  cannot  be  warmed,  nor  the  other  melted,  by 
such  rays.  The  cloth  is  white  and  the  snow  is  white,  be- 
cause their  confusedly  mingled  fibres  and  particles  are 
incompetent  to  absorb  the  luminous  rays.  Whether,  then, 
the  cloth  will  sink  or  not  depends  entirely  upon  the  dark 
rays  of  the  sun.  Now,  the  substance  which  absorbs  these 
dark  rays  with  the  greatest  avidity  is  ice — or  snow,  which 
is  merely  ice  in  powder.  Hence,  a  less  amount  of  heat 
will  be  lodged  in  the  cloth  than  in  the  surrounding  snow. 
The  cloth  must  therefore  act  as  a  shield  to  the  snow  on 
which  it  rests;  and,  in  consequence  of  the  more  rapid 
fusion  of  the  exposed  snow,  its  shield  must,  in  due  time, 
be  left  behind,  perched  upon  an  eminence  like  a  glacier- 
table. 

But  though  the  snow  transcends  the  cloth,  both  as  a 
radiator   and   absorber,   it  does   not  much  transcend  it. 


RADIANT  HEAT  AND   ITS   RELATIONS  97 

Cloth  is  very  powerful  in  both  these  respects.  Let  us 
now  turn  our  attention  to  the  piece  of  black  cloth,  the 
texture  and  fabric  of  which  I  assume  to  be  the  same  as 
that  of  the  white.  For  our  object  being  to  compare  the 
effects  of  color,  we  must,  in  order  to  study  this  effect  in 
its  purity,  preserve  all  the  other  conditions  constant.  Let 
us  then  suppose  the  black  cloth  to  be  obtained  from  the 
dyeing  of  the  white.  The  cloth  itself,  without  reference  to 
the  dye,  is  nearly  as  good  an  absorber  of  heat  as  the  snow 
around  it.  But  to  the  absorption  of  the  dark  solar  rays 
by  the  undyed  cloth  is  now  added  the  absorption  of  the 
whole  of  the  luminous  rays,  and  this  great  additional  in- 
flux of  heat  is  far  more  than  sufficient  to  turn  the  balance 
in  favor  of  the  black  cloth.  The  sum  of  its  actions  on  the 
dark  and  luminous  rays  exceeds  the  action  of  the  snow 
on  the  dark  rays  alone.  Hence  the  cloth  will  sink  in  the 
snow,  and  this  is  the  complete  analysis  of  Franklin's 
experiment. 

Throughout  this  discourse  the  main  stress  has  been  laid 
on  chemical  constitution,  as  influencing  most  powerfully 
the  phenomena  of  radiation  and  absorption.  With  regard 
to  gases  and  vapors,  and  to  the  liquids  from  which  these 
vapors  are  derived,  it  has  been  proved  by  the  most  varied 
and  conclusive  experiments  that  the  acts  of  radiation  and 
absorption  are  molecular — ^that  they  depend  upon  chemical, 
and  not  upon  mechanical,  condition.  In  attempting  to  ex- 
tend this  principle  to  solids  I  was  met  by  a  multitude  of 
facts,  obtained  by  celebrated  experimenters,  which  seemed 
flatly  to  forbid  such  an  extension.  Mellon i,  for  example, 
had  found  the  same  radiant  and  absorbent  power  for  chalk 
and  lamp-black.  MM.  Masson  and  Courtepee  had  per« 
formed  a  most  elaborate  series  of  experiments  on  chemi- 

SCIENCE — Y — 5 


98  FRAGMENTS   OF  SCIENCE 

cal  precipitates  of  various  kinds,  and  found  tliat  tliey  one 
&\id  all  manifested  the  same  power  of  radiation.  They 
concluded  from  their  researches,  that  when  bodies  are  re- 
duced to  an  extremely  fine  state  of  division,  the  influence 
of  this  state  is  so  powerful  as  entirely  to  mask  and  over- 
ride whatever  influence  may  be  due  to  chemical  consti- 
tution. 

But  it  appears  to  me  that  through  the  whole  of  these 
researches  an  oversight  has  run,  the  mere  mention  of 
which  will  show  what  caution  is  essential  in  the  opera- 
tions of  experimental  philosophy;  while  an  experiment  or 
two  will  make  clear  wherein  the  oversight  consists.  Fill- 
ing a  brightly  polished  metal  cube  with  boiling  water,  I 
determine  the  quantity  of  heat  emitted  by  two  of  the  bright 
surfaces.  As  a  radiator  of  heat  one  of  them  far  transcends 
the  other.  Both  surfaces  appear  to  be  metallic;  what,  then, 
is  the  cause  of  the  observed  difference  in  their  radiative 
power?  Simply  this:  one  of  the  surfaces  is  coated  with 
transparent  gum,  through  which,  of  course,  is  seen  the 
metallic  lustre  behind;  and  this  varnish,  though  so  per- 
fectly transparent  to  luminous  rays,  is  as  opaque  as  pitch, 
or  lamp-black,  to  non-luminous  ones.  It  is  a  powerful 
emitter  of  dark  rays;  it  is  also  a  powerful  absorber. 
While,  therefore,  at  the  present  moment,  it  is  copiously 
pouring  forth  radiant  heat  itself,  it  does  not  allow  a  single 
ray  from  the  metal  behind  to  pass  through  it.  The  varnish 
then,  and  not  the  metal,  is  the  real  radiator. 

Now,  Melloni,  and  Masson,  and  Courtepee,  experi- 
mented thus:  they  mixed  their  powders  and  precipitates 
with  gum- water,  and  laid  them,  by  means  of  a  brush,  upon 
the  surfaces  of  a  cube  like  this.  True,  they  saw  their  red 
powders  red,  their  white  ones  white,  and  their  black  ones 


RADIANT  HEAT  AND   ITS   RELATIONS  99 

black,  but  they  saw  tliese  colors  through  the  coat  of  varnish 
which  surrounded  every  particle.  When,  therefore,  it  was^ 
concluded  that  color  had  no  influence  on  radiation,  no 
chance  had  been  given  to  it  of  asserting  its  influence; 
when  it  was  found  that  all  chemical  precipitates  radiated 
alike,  it  was  the  radiation  from  a  varnish,  common  to  them 
all,  which  showed  the  observed  constancy.  Hundreds,  per- 
haps thousands,  of  experiments  on  radiant  heat  have  been 
performed  in  this  way,  by  various  inquirers,  but  the  work 
will,  I  fear,  have  to  be  done  over  again.  I  am  not,  in- 
deed, acquainted  with  an  instance  in  which  an  oversight 
of  so  trivial  a  character  has  been  committed  by  so  many 
able  men  in  succession,  vitiating  so  large  an  amount  of 
otherwise  excellent  work. 

Basing  our  reasonings  thus  on  demonstrated  facts,  we 
arrive  at  the  extremely  probable  conclusion  that  the  en- 
velope of  the  particles,  and  not  the  particles  themselves, 
was  the  real  radiator  in  the  experiments  just  referred  to. 
To  reason  thus,  and  deduce  their  more  or  less  probable 
consequences  from  experimental  facts,  is  an  incessant  ex- 
ercise of  the  student  of  physical  science.  But  having  thus 
followed,  for  a  time,  the  light  of  reason  alone  through  a 
series  of  phenomena,  and  emerged  from  them  with  a  purely 
intellectual  conclusion,  our  duty  is  to  bring  that  conclu- 
sion to  an  experimental  test.  In  this  way  we  fortify  our 
science. 

For  the  purpose  of  testing  our  conclusion  regarding  the 
influence  of  the  gum,  I  take  two  powders  presenting  the 
same  physical  appearance;  one  of  them  is  a  compound  of 
mercury,  and  the  other  a  compound  of  lead.  On  two  sur- 
faces of  a  cube  are  spread  these  bright  red  powders,  with- 
out varnish  of  any  kind.      Filling  the  cube  with  boiling 


100  FRAOMENTS   OF  SCIENCE 

water,  and  determining  the  radiation  from  the  two  surfaces, 
one  of  them  is  found  to  emit  thirty-nine  units  of  heat,  while 
the  other  emits  seventy -four.  This,  surely,  is  a  great  dif- 
ference. Here,  however,  is  a  second  cube,  having  two  of 
its  surfaces  coated  with  the  same  powders,  the  only  differ- 
ence being  that  the  powders  are  laid  on  by  means  of  a 
transparent  gum.  Both  surfaces  are  now  absolutely  alike 
in  radiative  power.  Both  of  them  emit  somewhat  more 
than  was  emitted  by  either  of  the  unvarnished  powders, 
simply  because  the  gum  employed  is  a  better  radiator  than 
either  of  them.  Excluding  all  varnish,  and  comparing 
white  with  white,  vast  differences  are  found;  comparing 
black  with  black,  they  are  also  different;  and  when  black 
and  white  are  compared,  in  some  cases  the  black  radiates 
far  more  than  the  white,  while  in  other  cases  the  white 
radiates  far  more  than  the  black.  Determining,  moreover, 
the  absorptive  power  of  those  powders,  it  is  found  to  go 
hand-in-hand  with  their  radiative  power.  The  good  radi- 
ator is  a  good  absorber,  and  the  bad  radiator  is  a  bad  ab- 
sorber. From  all  this  it  is  evident  that,  as  regards  the 
radiation  and  absorption  of  non -luminous  heat,  color  teaches 
us  nothing;  and  that  even  as  regards  the  radiation  of  the 
sun,  consisting  as  it  does  mainly  of  non-luminous  rays, 
conclusions  as  to  the  influence  of  color  may  be  altogether 
delusive.  This  is  the  strict  scientific  upshot  of  our  re- 
searches. But  it  is  not  the  less  true  that  in  the  case  of 
wearing  apparel — and  this  for  reasons  which  I  have  given 
in  analyzing  the  experiment  of  Franklin — black  dresses  are 
more  potent  than  white  ones  as  absorbers  of  solar  heat. 

Thus,  in  brief  outline,  have  been  brought  before  you  a 
few  of  the  results  of  recent  inquiry.  If  you  ask  me  what 
is  the  use  of  them,  I  can  hardly  answer  you,  unless  you 


RADIANT  HEAT  AND    ITS   RELATIONS  101 

define  the  term  use.  If  you  meant  to  ask  whetlier  those 
dark  rays  which  clear  away  the  Alpine  snows  will  ever  be 
applied  to  the  roasting  of  turkeys,  or  the  driving  of  steam- 
engines — while  affirming  their  power  to  do  both,  I  would 
frankly  confess  that  they  are  not  at  present  capable  of 
competing  profitably  with  coal  in  these  particulars.  Still 
they  may  have  great  uses  unknown  to  me;  and,  when  our 
coal-fields  are  exhausted,  it  is  possible  that  a  more  ethe- 
real race  than  we  are  may  cook  their  victuals,  and  perform 
their  work,  in  this  transcendental  way.  But  is  it  neces- 
sary that  the  student  of  science  should  have  his  labors 
tested  by  their  possible  practical  applications  ?  What  is 
the  practical  value  of  Homer's  Iliad?  You  smile,  and 
possibly  think  that  Homer's  Iliad  is  good  as  a  means  of 
culture.  There's  the.  rub.  The  people  who  demand  of  sci- 
ence practical  uses,  forget,  or  do  not  know,  that  it  also  is 
great  as  a  means  of  culture — that  the  knowledge  of  this 
wonderful  universe  is  a  thing  profitable  in  itself,  and 
requiring  no  practical  application  to  justify  its  pursuit. 

But,  while  the  student  of  Nature  distinctly  refuses  to 
have  his  labors  judged  by  their  practical  issues,  unless  the 
term  practical  be  made  to  include  mental  as  well  as  ma- 
terial good,  he  knows  full  well  that  the  greatest  practical 
triumphs  have  been  episodes  in  the  search  after  pure  natu- 
ral truth.  The  electric  telegraph  is  the  standing  wonder 
of  this  age,  and  the  men  whose  scientific  knowledge,  and 
mechanical  skill,  have  made  the  telegraph  what  it  is,  are 
deserving  of  all  honor.  In  fact,  they  have  had  their  re- 
ward, both  in  reputation  and  in  those  more  substantial 
benefits  which  the  direct  service  of  the  public  always  car- 
ries in  its  train.  But  who,  I  would  ask,  put  the  soul  into 
this  telegraphic  body?     Who   snatched   from   heaven   the 


102  FRAGMENTS    OF   SCIENCE 

fire  tliat  flashes  along  the  line  ?  This,  I  am  bound  to  say, 
was  done  by  two  men,  the  one  a  dweller  in  Italy,'  the 
other  a  dweller  in  England,'  who  never  in  their  inquiries 
consciously  set  a  practical  object  before  them — whose  only 
stimulus  was  the  fascination  which  draws  the  climber  to  a 
never-trodden  peak,  and  would  have  made  Caesar  quit  his 
victories  for  the  sources  of  the  Nile.  That  the  knowledge 
brought  to  us  by  those  prophets,  priests,  and  kings  of  sci- 
ence, is  what  the  world  calls  "useful  knowledge,"  the 
triumphant  application  of  their  discoveries  proves.  But 
science  has  another  function  to  fulfil,  in  the  storing  and 
the  training  of  the  human  mind;  and  I  would  base  my  ap- 
peal to  you  on  the  specimen  which  has  this  evening  been 
brought  before  you,  whether  any  system  of  education  at 
the  present  day  can  be  deemed  even  approximately  com- 
plete, in  which  the  knowledge  of  Nature  is  neglected  or 
ignored. 

»  Yolta.  *  Faraday. 


IV 


NEW   CHEMICAL  KEACTIONS   PRODUCED   BY   LIGHT 

1868-1869 


M^ 


B  ASURED  by  their  power,  not  to  excite  vision,  but 
to  produce  heat — in  other  words,  measured  by  thf^ir 
absolute  energy — the  ultra-red  waves  of  the  sun 
and  of  the  electric  light,  as  shown  in  the  preceding  arti- 
cles, far  transcend  the  visible.  In  the  domain  of  chem- 
istry, however,  there  are  numerous  cases  in  which  the 
more  powerful  waves  are  ineffectual,  while  the  more  mi- 
nute waves,  through  what  may  be  called  their  timeliness 
of  application,  are  able  to  produce  great  effects.  A  series 
of  these,  of  a  novel  and  beautiful  character,  discovered  in 
1868,  and  further  illustrated  in  subsequent  years,  may  be 
exhibited  by  subjecting  the  vapors  of  volatile  liquids  to 
the  action  of  concentrated  sunlight,  or  to  the  concentrated 
beam  of  the  electric  light.  Their  investigation  led  up  to 
the  discourse  on  "Dust  and  Disease,"  which  follows  in 
"this  volume;  and  for  this  reason  some  account  of  them 
is  introduced  here. 

A  glass  tube  three  feet  long  and  three  inches  wide, 
which  had  been  frequently  employed  in  my  researches  on 
radiant  heat,  was  supported  horizontally  on  two  stands. 
At  one  end  of  the  tube  was  placed  an  electric  lamp,  the 
height  and  position  of  both  being  so  arranged  that  the  axis 
of  the  tube,  and  that  of  the  beam  issuing  from  the  lamp, 

(103) 


104 


FRAGMENTS    OF  SCIENCE 


were  coincident.  In  tlie  first  experiments  the  two  ends  of 
tlie  tube  were  closed  by  plates  of  rock-salt,  and  subse- 
quently by  plates  of  glass.  For  tbe  sake  of  distinction, 
I  call  tbis  tube  the  experimental  tube.  It  was  connected 
witb  an  air-pump,  and  also  with  a  series  of  drying  and 
other  tubes  used  for  the  purification  of  the  air. 

A  number  of  test-tubes,  like  f.  Fig.  2  (I  have  used  at 
least  fifty  of  them),  were  converted  into  Woulf's  flasks. 
ijj  Each  of  them  was  stopped  by  a  cork, 
through  which  passed  two  glass  tubes: 
one  of  these  tubes  (a)  ended  immedi- 
ately below  the  cork,  while  the  other 
{b)  descended  to  the  bottom  of  the 
flask,  being  drawn  out  at  its  lower 
end  to  an  orifice  about  0*03  of  an 
inch  in  diameter.  It  was  found  nec- 
essary to  coat  the  cork  carefully  with 
cement.  In  the  later  experiments 
corks  of  vulcanized  India-rubber  were 
invariably  employed. 

The  little  flask,  thus  formed,  being 
partially  filled  with  the  liquid  whose 
vapor  was  to  be  examined,  was  intro- 
duced into  the  path  of  the  purified 
current  of  air.  The  experimental  tube 
being  exhausted,  and  the  cock  which 
Fio.2.  cut  off  the  supply  of  purified  air  be- 

ing cautiously  turned  on,  the  air  entered  the  flask  through 
the  tube  b,  and  escaped  by  the  small  orifice  at  the  lower 
end  of  b  into  the  liquid.  Through  this  it  bubbled,  loading 
itself  with  vapor,  after  which  the  mixed  air  and  vapor, 
passing  from  the  flask  by  the  tube  a,  entered  the  experi- 


DECOMPOSITION   BY  LIGHT  105 

mental  tube,  where  they  were  subjected  to  the  action  of 
light. 

The  whole  arrangement  is  shown  in  Fig.  3,  where  L  rep- 
resents the  electric  lamp,  s  s'  the  experimental  tube,  p  p' 
the  pipe  leading  to  the  air-pump,  and  F  the  test-tube  con- 
taining the  volatile  liquid.  The  tube  t  i'  is  plugged  with 
cotton-wool  intended  to  intercept  the  floating  matter  of  the 
air;  the  bent  tube  t'  contains  caustic  potash,  the  tube  T 
sulphuric  acid,  the  one  intended  to  remove  the  carbonic 
acid  and  the  other  the  aqueous  vapor  of  the  air. 

The  power  of  the  electric  beam  to  reveal  the  existence 
of  anything  within  the  experimental  tube,  or  the  impuri- 
ties of  the  tube  itself,  is  extraordinary.  When  the  ex- 
periment is  made  in  a  darkened  room,  a  tube  which  in 
ordinary  daylight  appears  absolutely  clean  is  often  shown 
by  the  present  mode  of  examination  to  be  exceedingly 
filthy. 

The  following  are  some  of  the  results  obtained  with 
this  arrangement: 

Nitrite  of  Amyl. — The  vapor  of  this  liquid  was,  in  the 
first  instance,  permitted  to  enter  the  experimental  tube 
while  the  beam  from  the  electric  lamp  was  passing 
through  it.  Curious  clouds,  the  cause  of  which  was 
then  unknown,  were  observed  to  form  near  the  place 
of  entry,  being  afterward  whirled  through  the  tube. 

The  tube  being  again  exhausted,  the  mixed  air  and 
vapor  were  allowed  to  enter  it  in  the  dark.  The  slightly 
convergent  beam  of  the  electric  light  was  then  sent  through 
the  mixture.  For  a  moment  the  tube  was  optically  empty^ 
nothing  whatever  being  seen  within  it;  but  before  a  second 
had  elapsed  a  shower  of  particles  was  precipitated  on  the 
beam.     The  cloud   thus   generated   became  denser  as   the 


lOfS 


FRAGMENTS   OF  SCIENCE 


DECOMPOSITION  BY  LmHT  lOT 

light  continued  to  act,  showing  at  some  places  vivid 
iridescence. 

The  lens  of  the  electric  lamp  was  now  placed  so  as 
to  form  within  the  tube  a  strongly  convergent  cone  of 
rays.  The  tube  was  cleansed  and  again  filled  in  dark- 
ness. When  the  light  was  sent  through  it,  the  precipi- 
tation upon  the  beam  was  so  rapid  and  intense  that  the 
cone,  which  a  moment  before  was  invisible,  flashed  sud- 
denly forth  like  a  solid  luminous  spear.  The  effect  was 
the  same  when  the  air  and  vapor  were  allowed  to  enter 
the  tube  in  diffuse  daylight.  The  cloud,  however,  which 
shone  with  such  extraordinary  radiance  under  the  electric 
beam,  was  invisible  in  the  ordinary  light  of  the  labora- 
tory. 

The  quantity  of  mixed  air  and  vapor  within  the  experi- 
mental tube  could,  of  course,  be  regulated  at  pleasure. 
The  rapidity  of  the  action  diminished  with  the  attenua- 
tion of  the  vapor.  When,  for  example,  the  mercurial  col- 
umn associated  with  the  experimental  tube  was  depressed 
only  five  inches,  the  action  was  not  nearly  so  rapid  as 
when  the  tube  was  full.  In  such  cases,  however,  it  was 
exceedingly  interesting  to  observe,  after  some  seconds  of 
waiting,  a  thin  streamer  of  delicate  bluish-white  cloud 
slowly  forming  along  the  axis  of  the  tube,  and  finally 
swelling  so  as  to  fill  it. 

When  dry  oxygen  was  employed  to  carry  in  the  vapor, 
the  effect  was  the  same  as  that  obtained  with  air. 

When  dry  hydrogen  was  used  as  a  vehicle,  the  effect 
was  also  the  same. 

The  effect,  therefore,  is  not  due  to  any  interaction 
between  the  vapor  of  the  nitrite  and  its  vehicle. 

This  was  further  demonstrated   by  the  deportment  of 


108  FRAGMENTS   OF  SCIENCE 

the  vapor  itself.  When  it  was  permitted  to  enter  the  ex- 
perimental tube  unmixed  with  air  or  any  other  gas,  the 
effect  was  substantially  the  same.  Hence  the  seat  of  the 
observed  action  is  the  vapor. 

This  action  is  not  to  be  ascribed  to  heat.  As  regards 
the  glass  of  the  experimental  tube,  and  the  air  within  the 
tube,  the  beam  employed  in  these  experiments  was  per- 
fectly cold.  It  had  been  sifted  by  passing  it  through  a 
solution  of  alum  and  through  the  thick  double- convex  lens 
of  the  lamp.  When  the  unsifted  beam  of  the  lamp  was 
employed,  the  effect  was  still  the  same;  the  obscure  calo- 
rific rays  did  not  appear  to  interfere  with  the  result. 

My  object  here  being  simply  to  point  out  to  chemists 
a  method  of  experiment  which  reveals  a  new  and  beau- 
tiful series  of  reactions,  I  left  to  them  the  examination 
of  the  products  of  decomposition.  The  group  of  atoms 
forming  the  molecule  of  nitrite  of  amyl  is  obviously 
shaken  asunder  by  certain  specific  waves  of  the  electric 
beam,  nitric  oxide  and  other  products,  of  which  the  ni- 
trate of  amyl  is  probably  one,  being  the  result  of  the 
decomposition.  The  brown  fumes  of  nitrous  acid  were 
seen  mingling  with  the  cloud  within  the  experimental 
tube.  The  nitrate  of  amyl,  being  less  volatile  than  the 
nitrite,  and  not  being  able  to  maintain  itself  in  the  con- 
dition of  vapor,  would  be  precipitated  as  a  visible  cloud 
along  the  track  of  the  beam. 

In  the  anterior  portions  of  the  tube  a  powerful  sifting 
of  the  beam  by  the  vapor  occurs,  which  diminishes  the 
chemical  action  in  the  posterior  portions.  In  some  ex- 
periments the  precipitated  cloud  only  extended  half-way 
down  the  tube.  When,  under  these  circumstances,  .the 
lamp   was   shifted   so   as   to  send  the   beam   through    the 


DECOMPOSITION  BY  LIGHT  109 

other  end  of  the  tube,  copious  precipitation  occurred  there 
also. 

Solar  light  also  effects  the  decomposition  of  the  nitrite- 
of-amyl  vapor.  On  October  10,  1868,  I  partially  dark- 
ened a  small  room  in  the  Eojal  Institution,  into  which 
the  sun  shone,  permitting  the  light  to  enter  through  an 
open  portion  of  the  window- shutter.  In  the  track  of  the 
beam  was  placed  a  large  plano-convex  lens,  which  formed 
a  fine  convergent  cone  in  the  dust  of  the  room  behind  it. 
The  experimental  tube  was  filled  in  the  laboratory,  cov- 
ered with  a  black  cloth,  and  carried  into  the  partially 
darkened  room.  On  thrusting  one  end  of  the  tube  into 
the  cone  of  rays  behind  the  lens,  precipitation  within  the 
cone  was  copious  and  immediate.  The  vapor  at  the  dis- 
tant end  of  the  tube  was  in  part  shielded  by  that  in  front, 
and  was  also  more  feebly  acted  on  through  the  divergence 
of  the  rays.  On  reversing  the  tube,  a  second  and  similar 
cone  was  precipitated. 

Physical  Considerations 

I  sought  to  determine  the  particular  portion  of  the 
light  which  produced  the  foregoing  effects.  When,  pre- 
vious to  entering  the  experimental  tube,  the  beam  was 
caused  to  pass  through  a  red  glass,  the  effect  was  greatly 
weakened,  but  not  extinguished.  This  was  also  the  case 
with  various  samples  of  yellow  glass.  A  blue  glass  being 
introduced  before  the  removal  of  the  yellow  or  the  red, 
on  taking  the  latter  away  prompt  precipitation  occurred 
along  the  track  of  the  blue  beam.  Hence,  in  this  case, 
the  more  refrangible  rays  are  the  most  chemically  active. 
The  color  of  the  liquid  nitrite  of  amyl  indicates  that  this 
must   be   the  case;  it  is  a  feeble  but  distinct  yellow:    ia 


llO  FRAGMENTS   OF  SCIENCE 

Other  words,  the  yellow  portion  of  the  beam  is  most  freely 
transmitted.  It  is  not,  however,  the  transmitted  portion 
of  any  beam  which  produces  chemical  action,  but  the  ab- 
sorbed portion.  Blue,  as  the  complementary  color  to  yel- 
low, is  here  absorbed,  and  hence  the  more  energetic  action 
of  the  blue  rays. 

This  reasoning,  however,  assumes  that  the  same  rays 
are  absorbed  by  the  liquid  and  its  vapor.  The  assump- 
tion is  worth  testing.  A  solution  of  the  yellow  chromate 
of  potash,  the  color  of  which  may  be  made  almost,  if  not 
altogether,  identical  with  that  of  the  liquid  nitrite  of  amyl, 
was  found  far  more  effective  in  stopping  the  chemical 
rays  than  either  the  red  or  the  yellow  glass.  But  of  all 
substances  the  liquid  nitrite  itself  is  most  potent  in  arrest- 
ing the  rays  which  act  upon  its  vapor.  A  layer  one- 
eighth  of  an  inch  in  thickness,  which  scarcely  perceptibly 
affected  the  luminous  intensity,  absorbed  the  entire  chem- 
ical energy  of  the  concentrated  beam  of  the  electric  light. 

The  close  relation  subsisting  between  a  liquid  and  its 
vapor,  as  regards  their  action  upon  radiant  heat,  has  been 
already  amply  demonstrated.'  As  regards  the  nitrite  of 
amyl,  this  relation  is  more  specific  than  in  the  cases 
hitherto  adduced;  for  here  the  special  constituent  of  the 
beam,  which  provokes  the  decomposition  of  the  vapor,  is 
shown  to  be  arrested  by  the  liquid. 

A  question  of  extreme  importance  in  molecular  physics 
here  arises:  What  is  the  real  mechanism  of  this  absorp- 
tion,  and   where   is   its   seat?'     I   figure,   as   others   do,   a 


>  "Phil.  Trans."  1864;  "Heat,  a  Mode  of  Motion,"  chap.  xii. ;  and  p.  6t 
of  this  volume. 

'  My  attention  was  very  forcibly  directed  to  this  subject  some  years  ago  by 
a  conversation  with  my  excellent  friend  Professor  Clausius. 


DECOMPOSITION  BY  LIGHT  111 

molecule  as  a  group  of  atoms,  held  together  by  their 
mutual  forces,  but  still  capable  of  motion  among  them- 
selves. The  vapor  of  the  nitrite  of  amyl  is  to  be  regarded 
as  an  assemblage  of  such  molecules.  The  question  now 
before  us  is  this :  In  the  act  of  absorption,  is  it  the 
molecules  that  are  effective,  or  is  it  their  constituent 
atoms?  Is  the  vis  viva  of  the  intercepted  light- waves 
transferred  to  the  molecule  as  a  whole,  or  to  its  con- 
stituent parts? 

The  molecule,  as  a  whole,  can  only  vibrate  in  virtue 
of  the  forces  exerted  between  it  and  its  neighbor  mole- 
cules. The  intensity  of  these  forces,  and  consequently  the 
rate  of  vibration,  would,  in  this  case,  be  a  function  of  the 
distance  between  the  molecules.  Now  the  identical  ab- 
sorption of  the  liquid  and  of  the  vaporous  nitrite  of  amyl 
indicates  an  identical  vibrating  period  on  the  part  of 
liquid  and  vapor,  and  this,  to  my  mind,  amounts  to  an 
experimental  proof  that  the  absorption  occurs  in  the  main 
within  the  molecule.  For  it  can  hardly  be  supposed,  if 
the  absorption  were  the  act  of  the  molecule  as  a  whole, 
that  it  could  continue  to  affect  waves  of  the  same  period 
after  the  substance  had.  passed  from  the  vaporous  to  the 
liquid  state. 

In  point  of  fact,  the  decomposition  of  the  nitrite  of 
amyl  is  itself  to  some  extent  an  illustration  of  this  in- 
ternal molecular  absorption;  for  were  the  absorption  the 
act  of  the  molecule  as  a  whole,  the  relative  motions  of  its 
constituent  atoms  would  remain  unchanged,  and  there 
would  be  no  mechanical  cause  for  their  separation.  It  is 
probably  the  synchronism  of  the  vibrations  of  one  portion 
of  the  molecule  with  the  incident  waves  that  enables  the 
amplitude  of  those  vibrations  to  augment,  until  the  chain 


112  FRAGMENTS   OF  SCIENCE 

which  binds  the  parts  of  the  molecule  together  is  snapped 
asunder. 

I  anticipate  wide,  if  not  entire,  generality  for  the  fact 
that  a  liquid  and  its  vapor  absorb  the  same  rays.  A  cell 
of  liquid  chlorine  would,  I  imagine,  deprive  light  more 
effectually  of  its  power  of  causing  chlorine  and  hydrogen 
to  combine  than  any  other  filter  of  the  luminous  rays. 
The  rays  which  give  chlorine  its  color  have  nothing  to  do 
with  this  combination,  those  that  are  absorbed  by  the 
chlorine  being  the  really  effective  rays.  A  highly  sensi- 
tive bulb,  containing  chlorine  and  hydrogen,  in  the  exact 
proportions  necessary  for  the  formation  of  hydrochloric 
acid,  was  placed  at  one  end  of  an  experimental  tube,  the 
beam  of  the  electric  lamp  being  sent  through  it  from  the 
other.  The  bulb  did  not  explode  when  the  tube  was 
filled  with  chlorine,  while  the  explosion  was  violent  and 
immediate  when  the  tube  was  filled  with  air.  I  anticipate 
for  the  liquid  chlorine  an  action  similar  to,  but  still  more 
energetic  than,  that  exhibited  by  the  gas.  If  this  should 
prove  to  be  the  case,  it  will  favor  the  view  that  chlorine 
itself  is  molecular  and  not  monatomic. 

Production  of  Sky-blue   hy   the   Decomposition   of  Nitrite 

of  Amyl 

When  the  quantity  of  nitrite  vapor  is  considerable,  and 
the  light  intense,  the  chemical  action  is  exceedingly  rapid, 
the  particles  precipitated  being  so  large  as  to  whiten  the 
luminous  beam.  Not  so,  however,  when  a  well-mixed  and 
highly  attenuated  vapor  fills  the  experimental  tube.  The 
effect  now  to  be  described  was  first  obtained  when  the 
vapor  of  the  nitrite  was  derived  from  a  portion  of  its 
liquid  which   had   been   accidentally  introduced  into  the 


I 


DECOMPOSITION  BY  LIGHT  113 

passage  through  which  the  dry  air  flowed  into  the  experi- 
mental tube. 

In  this  case,  the  electric  beam  traversed  the  tube  for 
several  seconds  before  any  action  was  visible.  Decomposi- 
tion then  visibly  commenced,  and  advanced  slowly.  When 
the  light  was  very  strong,  the  cloud  appeared  of  a  milky 
blue.  When,  on  the  contrary,  the  intensity  was  moderate, 
the  blue  was  pure  and  deep.  In  Briicke's  important  ex- 
periments on  the  blue  of  the  sky  and  the  morning  and 
evening  red,  pure  mastic  is  dissolved  in  alcohol,  and  then 
dropped  into  water  well  stirred.  When  the  proportion  of 
mastic  to  alcohol  is  correct,  the  resin  is  precipitated  so 
finely  as  to  elude  the  highest  microscopic  power.  By 
reflected  light,  such  a  medium  appears  bluish,  by  trans- 
mitted light  yellowish,  which  latter  color,  by  augmenting 
the  quantity  of  the  precipitate,  can  be  caused  to  pass  into 
orange  or  red. 

But  the  development  of  color  in  the  attenuated  nitrite- 
of-amyl  vapor  is  doubtless  more  similar  to  what  takes 
place  in  our  atmosphere.  The  blue,  moreover,  is  far 
purer  and  more  sky-like  than  that  obtained  from  Briicke's 
turbid  medium.  Never,  even  in  the  skies  of  the  Alps, 
have  I  seen  a  richer  or  a  purer  blue  than  that  attainable 
by  a  suitable  disposition  of  the  light  falling  upon  the  pre- 
cipitated vapor. 

Iodide  of  Allyl — Among  the  liquids  hitherto  subjected 
to  the  concentrated  electric  light,  iodide  of  allyl,  in  point 
of  rapidity  and  intensity  of  action,  comes  next  to  the 
nitrite  of  amyl.  With  the  iodide  I  have  employed  both 
oxygen  and  hydrogen,  as  well  as  air,  as  a  vehicle,  and 
found  the  effect  in  all  cases  substantially  the  same.  The 
cloud-column  here  was  exquisitely  beautiful.     It  revolved 


114  FRAGMENTS   OF  SCIENCE 

round  the  axis  of  the  decomposing  beam;  it  was  nipped 
at  certain  places  like  an  hour-glass,  and  round  the  two 
bells  of  the  glass  delicate  cloud-filaments  twisted  them- 
selves in  spirals.  It  also  folded  itself  into  convolutions 
resembling  those  of  shells.  In  certain  conditions  of  the 
atmosphere  in  the  Alps  I  have  often  observed  clouds  of 
a  special  pearly  lustre;  when  hydrogen  was  made  the 
vehicle  of  the  iodide- of- ally  1  vapor  a  similar  lustre  was 
most  exquisitely  shown.  With  a  suitable  disposition  of 
the  light,  the  purple  hue  of  iodine-vapor  came  out  very 
strongly  in  the  tube. 

The  remark  already  made,  as  to  the  bearing  of  the 
decomposition  of  nitrite  of  amyl  by  light  on  the  question 
of  molecular  absorption,  applies  here  also;  for  were  the 
absorption  the  work  of  the  molecule  as  a  whole,  the  iodine 
would  not  be  dislodged  from  the  allyl  with  which  it  is 
combined.  The  non-synchronism  of  iodine  with  the  waves 
of  obscure  heat  is  illustrated  by  its  marvellous  transpar- 
ency to  such  heat.  May  not  its  synchronism  with  the 
waves  of  light  in  the  present  instance  be  the  cause  of  its 
divorce  from  the  allyl? 

Iodide  of  IsopropyL — The  action  of  light  upon  the  va- 
por of  this  liquid  is,  at  first,  more  languid  than  upon 
iodide  of  allyl;  indeed  many  beautiful  reactions  may  be 
overlooked,  in  consequence  of  this  languor  at  the  com- 
mencement. After  some  minutes'  exposure,  however, 
clouds  begin  to  form,  which  grow  in  density  and  in 
beauty  as  the  light  continues  to  act.  In  every  experiment 
hitherto  made  with  this  substance  the  column  of  cloud 
filling  the  experimental  tube  was  divided  into  two  dis- 
tinct parts  near  the  middle  of  the  tube.  In  one  experi- 
ment a  globe  of  cloud  formed  at  the  centre,  from  which, 


DECOMPOSITION  BY  LIGHT  115 

light  and  left,  issued  an  axis  uniting  the  globe  with  two 
adjacent  cylinders.  Both  globe  and  cylinders  were  ani- 
mated by  a  common  motion  of  rotation.  As  the  action 
continued,  paroxysms  of  motion  were  manifested;  the  vari- 
ous parts  of  the  cloud  would  rush  through  each  other  with 
sudden  violence.  During  these  motions  beautiful  and  gro« 
tesque  cloud-forms  were  developed.  At  some  places  the 
nebulous  mass  would  become  ribbed  so  as  to  resemble  the 
graining  of  wood;  a  longitudinal  motion  would  at  times 
generate  in  it  a  series  of  curved  transverse  bands,  the 
retarding  influence  of  the  sides  of  the  tube  causing  an  ap- 
pearance resembling,  on  a  small  scale,  the  dirt-bands  of 
the  Mer  de  Glace.  In  the  anterior  portion  of  the  tube 
those  sudden  commotions  were  most  intense;  here  buds 
of  cloud  would  sprout  forth,  and  grow  in  a  few  seconds 
into  perfect  flower-like  forms.  The  cloud  of  iodide  of 
isopropyl  had  a  character  of  its  own,  and  differed  materi- 
ally from  all  others  that  I  had  seen.  A  gorgeous  mauve 
color  was  observed  in  the  last  twelve  inches  of  the  tube; 
the  vapor  of  iodine  was  present,  and  it  may  have  been 
the  sky-blue  scattered  by  the  precipitated  particles  which, 
mingling  with  the  purple  of  the  iodine,  produced  the 
mauve.  As  in  all  other  cases  here  adduced,  the  effects 
were  proved  to  be  due  to  the  light;  they  never  occurred 
in  darkness. 

The  forms  assumed  by  some  of  those  actinic  clouds,  as 
T  propose  to  call  them,  in  consequence  of  rotations  and 
other  motions,  due  to  differences  of  temperature,  are  per- 
fectly astounding.  I  content  myself  here  with  a  meagre 
description  of  one  more  of  them. 

The  tube  being  filled  with  the  sensitive  mixture,  the 
beam  was  sent  through  it,  the  lens  at  the  same  time  being 


Xl6  FRAGMENTS   OF  SCIENCE 

SO  placed  as  to  produce  a  cone  of  very  intense  light. 
Two  minutes  elapsed  before  anything  was  visible;  but  at 
the  end  of  this  time  a  faint  bluish  cloud  appeared  to  hang 
itself  on  the  most  concentrated  portion  of  the  beam. 

Soon  afterward  a  second  cloud  was  formed  five  inches 
further  down  the  experimental  tube.  Both  clouds  were 
united  by  a  slender  cord  of  the  same  bluish  tint  as  them- 
selves. 

As  the  action  of  the  light  continued,  the  first  cloud 
gradually  resolved  itself  into  a  series  of  parallel  disks  of 
exquisite  delicacy,  which  rotated  round  an  axis  perpen- 
dicular to  their  surfaces,  and  finally  blended  to  a  screw 
surface  with  an  inclined  generatrix.  This  gradually 
changed  into  a  filmy  funnel,  from  the  narrow  end  of 
which  the  '*cord''  extended  to  the  cloud  in  advance. 
The  latter  also  underwent  slow  but  incessant  modification. 
It  first  resolved  itself  into  a  series  of  strata  resembling 
those  of  the  electric  discharge.  After  a  little  time,  and 
through  changes  which  it  was  difficult  to  follow,  both 
clouds  presented  the  appearance  of  a  series  of  concentric 
funnels  set  one  within  the  other,  the  interior  ones  being 
seen  through  the  outer  ones.  Those  of  the  distant  cloud 
resembled  claret-glasses  in  shape.  As  many  as  six  fun- 
nels were  thus  concentrically  set  together,  the  two  series 
being  united  by  the  delicate  cord  of  cloud  already  re- 
ferred to.  Other  cords  and  slender  tubes  were  afterward 
formed,  which  coiled  themselves  in  delicate  spirals  around 
the  funnels. 

Eendering  the  light  along  the  connecting-cord  more 
intense,  it  diminished  in  thickness  and  became  whiter; 
this  was  a  consequence  of  the  enlargement  of  its  particles. 
The  cord   finally  disappeared,    while   the    funnels    melted 


ARTIFICIAL   SKY  117 

into  two  ghost-like  films,  shaped  like  parasols.  Thej 
were  barely  visible,  being  of  an  exceedingly  delicate  blue 
tint.  They  seemed  woven  of  blue  air.  To  compare  them 
with  cobweb  or  with  gauze  would  be  to  liken  them  to 
something  infinitely  grosser  than  themselves. 

In  all  cases  a  distant  candle-flame,  when  looked  at 
through  the  cloud,  was  sensibly  undimmed. 

§  2.  On  the  Blue  Color  of  the  Skt,  and  the 
Polarization  op  Sktlight' 

1869 

After  the  communication  to  the  Eoyal  Society  of  the 
foregoing  brief  account  of  a  new  Series  of  Chemical  Reac- 
tions produced  by  Light,  the  experiments  upon  this  sub- 
ject were  continued,  the  number  of  substances  thus  acted 
on  being  considerably  increased. 

I  now,  however,  beg  to  direct  attention  to  two  ques- 
tions glanced  at  incidentally  in  the  preceding  pages — ^the 
blue  color  of  the  sky,  and  the  polarization  of  skylight 
Reserving  the  historic  treatment  of  the  subject  for  a  more 
fitting  occasion,  I  would  merely  mention  now  that  these 
questions  constitute,  in  the  opinion  of  our  most  eminent 
authorities,  the  two  great  standing  enigmas  of  meteor- 
ology. Indeed  it  was  the  interest  manifested  in  them  by 
Sir  John  Herschel,  in  a  letter  of  singular  speculative 
power,  addressed  to  myself,  that  caused  me  to  enter  upon 
the  consideration  of  these  questions  so  soon. 

The  apparatus  with  which  I  work  consists,  as  already 
stated,  of  a  glass  tube  about  a  yard  in  length,  and  from 

*  In  my  ''Lectures  on  Light*'  (Longmans),  the  polarization  of  light  will  be 
found  briefly,  but,  X  trust  clearly  explained. 


118  FRAGMENTS    OF   SCIENCE 

2>^  to  3  inclies  internal  diameter.  The  vapor  to  be  ex- 
amined is  introduced  into  tkis  tube  in  tlie  manner  already 
described,  and  upon  it  the  condensed  beam  of  the  electric 
lamp  is  permitted  to  act,  until  the  neutrality  or  the  ac- 
tivity of  the  substance  has  been  declared. 

It  has  hitherto  been  my  aim  to  render  the  chemical 
action  of  light  upon  vapors  visible.  For  this  purpose 
substances  have  been  chosen,  one  at  least  of  whose  prod- 
ucts of  decomposition  under  light  shall  have  a  boiling- 
point  so  high,  that  as  soon  as  the  substance  is  formed  it 
shall  be  precipitated.  By  graduating  the  quantity  of  the 
vapor,  this  precipitation  may  be  rendered  of  any  degree 
of  fineness,  forming  particles  distinguishable  by  the  naked 
eye,  or  far  beyond  the  reach  of  our  highest  microscopic 
powers.  I  have  no  reason  to  doubt  that  particles  may  be 
thus  obtained,  whose  diameters  constitute  but  a  small 
fraction  of  the  length  of  a  wave  of  violet  light. 

In  all  cases  when  the  vapors  of  the  liquids  employed 
are  sufficiently  attenuated,  no  matter  what  the  liquid  may 
be,  the  visible  action  commences  with  the  formation  of  a 
blue  cloud.  But  here  I  must  guard  myself  against  all  mis- 
conception as  to  the  use  of  this  term.  The  "cloud"  here 
referred  to  is  totally  invisible  in  ordinary  daylight.  To 
be  seen,  it  requires  to  be  surrounded  by  darkness,  it  only 
being  illuminated  by  a  powerful  beam  of  light.  This  blue 
cloud  differs  in  many  important  particulars  from  the  finest 
ordinary  clouds,  and  might  justly  have  assigned  to  it  an 
intermediate  position  between  such  clouds  and  true  vapor. 
With  this  explanation,  the  term  ** cloud,"  or  "incipient 
cloud,**  or  "actinic  cloud,**  as  I  propose  to  employ  it, 
cannot,  1  think,  be  misunderstood. 

I  had   been   endeavoring  to   decompose  carbonic    acid 


ARTIFICIAL    SKY  119 

gas  by  light.  A  faint  bluish  cloud,  due  it  may  be,  or  it 
may  not  be,  to  the  residue  of  some  vapor  previously  em- 
ployed, was  formed  in  the  experimental  tube.  On  look- 
ing across  this  cloud  through  a  Nicol's  prism,  the  line 
of  vision  being  horizontal,  it  was  found  that  when  the 
short  diagonal  of  the  prism  was  vertical  the  quantity  of 
light  reaching  the  eye  was  greater  than  when  the  long 
diagonal  was  vertical.  When  a  plate  of  tourmaline  was 
held  between  the  eye  and  the  bluish  cloud,  the  quantity 
of  light  reaching  the  eye  when  the  axis  of  the  prism  was 
perpendicular  to  the  axis  of  the  illuminating  beam  was 
greater  than  when  the  axes  of  the  crystal  and  of  the  beam 
were  parallel  to  each  other. 

This  was  the  result  all  round  the  experimental  tube. 
Causing  the  crystal  of  tourmaline  to  revolve  round  the 
tube,  with  its  axis  perpendicular  to  the  illuminating  beam, 
the  quantity  of  light  that  reached  the  eye  was  in  all  its 
positions  a  maximum.  When  the  crystallographic  axis 
was  parallel  to  the  axis  of  the  beam,  the  quantity  of  light 
transmitted  by  the  crystal  was  a  minimum.  From  the 
illuminated  bluish  cloud,  therefore,  polarized  light  was 
discharged,  the  direction  of  maximum  polarization  being 
at  right  angles  to  the  illuminating  beam;  the  plane  of 
vibration  of  the  polarized  light  was  perpendicular  to  the 
beam.  * 

Thin  plates  of  selenite   or  of  quartz,   placed  between 

the  Nicol  and  the  actinic  cloud,    displayed  the  colors  of 

polarized  light,  these  colors  being  most  vivid  when   the 

I  — -^ 

*  This  is  still  an  undecided  point;  but  the  probabilities  are  ao  much  in  its 
fevor,  and  it  is  in  my  opinion  so  much  preferable  to  have  a  physical  image  on 
which  the  mind  can  rest,  that  I  do  not  hesitate  to  employ  the  phraseology  in 
the  text. 


120  FRAGMENTS   OF  SCIENCE 

line  of  vision  was  at  riglit  angles  to  the  experimental 
tube.  The  plate  of  selenite  usually  employed  was  a  cir- 
cle, thinnest  at  the  centre,  and  augmenting  uniformly  in 
thickness  from  the  centre  outward.  When  placed  in  ita 
proper  position  between  the  Nicol  and  the  cloud,  it  ex- 
hibited a  system  of  splendidly- colored  rings. 

The  cloud  here  referred  to  was  the  first  operated  upon 
in  the  manner  described.  It  may,  however,  be  greatly 
improved  upon  by  the  choice  of  proper  substances,  and 
by  the  application,  in  proper  quantities,  of  the  substances 
chosen.  Benzol,  bisulphide  of  carbon,  nitrite  of  amyl, 
nitrite  of  butyl,  iodide  of  allyl,  iodide  of  isopropyl,  and 
many  other  substances  may  be  employed.  I  will  take  the 
nitrite  of  butyl  as  illustrative  of  the  means  adopted  to 
secure  the  best  result,  with  reference  to  the  present  ques- 
tion. 

And  here  it  may  be  mentioned  that  a  vapor,  which 
when  alone,  or  mixed  with  air  in  the  experimental  tube, 
resists  the  action  of  light,  or  shows  but  a  feeble  result  of 
this  action,  may,  when  placed  in  proximity  with  another 
gas  or  vapor,  exhibit  vigorous,  if  not  violent  action.  The 
case  is  similar  to  that  of  carbonic  acid  gas,  which,  diffused 
in  the  atmosphere,  resists  the  decomposing  action  of  solar 
light,  but  when  placed  in  contiguity  with  chlorophyl  in 
the   leaves   of    plants    has   its   molecules    shaken   asunder. 

Dry  air  was  permitted  to  bubble  through  the  liquid 
nitrite  of  butyl,  until  the  experimental  tube,  which  had 
been  previously  exhausted,  was  filled  with  the  mixed  air 
and  vapor.  The  visible  action  of'  light  upon  the  mixture 
after  fifteen  minutes'  exposure  was  slight.  The  tube  was 
afterward  filled  with  half  an  atmosphere  of  the  mixed  air 
and  vapor,  and  a  second  half- atmosphere  of  air  which  had 


ABTIFICIAL  SKY  121 

been  permitted  to  bubble  through  fresh  commercial  hy- 
drochloric acid.  On  sending  the  beam  through  this  mixt- 
ure, the  tube,  for  a  moment,  was  optically  empty.  But 
the  pause  amounted  only  to  a  small  fraction  of  a  second, 
a  dense  cloud  being  immediately  precipitated  upon  the 
beam. 

This  cloud  began  blue,  but  the  advance  to  whiteness 
was  so  rapid  as  almost  to  justify  the  application  of  the 
term  instantaneous.  The  dense  cloud,  looked  at  perpen- 
dicularly to  its  axis,  showed  scarcely  any  signs  of  polar- 
ization.    Looked  at  obliquely,  the  polarization  was  strong. 

TJie  experimental  tube  being  again  cleansed  and  ex- 
hausted, the  mixed  air  and  nitrite-of- butyl  vapor  was  per- 
mitted to  enter  it  until  the  associated  mercury  column  wad 
depressed  tAt  of  an  inch.  In  other  words,  the  air  and 
vapor,  united,  exercised  a  pressure  not  exceeding  th  of 
an  atmosphere.  Air,  passed  through  a  solution  of  hydro- 
chloric acid,  was  then  added,  till  the  mercury  column  was 
depressed  three  inches.  The  condensed  beam  of  the  elec- 
tric light  was  passed  for  some  time  through  this  mixture 
without  revealing  anything  within  the  tube  competent  to 
scatter  the  light.  Soon,  however,  a  superbly  blue  cloud 
was  formed  along  the  track  of  the  beam,  and  it  continued 
blue  sufficiently  long  to  permit  of  its  thorough  examina- 
tion. The  light  discharged  from  the  cloud,  at  right  angles 
to  its  own  length,  was  at  first  perfectly  polarized.  It 
could  be  totally  quenched  by  the  Nicol.  By  degrees  the 
cloud  became  of  whitish  blue,  and  for  a  time  the  selenite 
colors,  obtained  by  looking  at  it  normally,  were  exceed- 
ingly brilliant.  The  direction  of  maximum  polarization 
was  distinctly  at  right  angles  to  the  illuminating  beam. 
This  continued  to  be  the  case  as  long  as  the  cloud  main- 

SCIENOE—  —6 


122  FRAGMENTS   OF  SCIENCE 

tained  a  decided  blue  color,  and  even  for  some  time  after 
the  blue  had  changed  to  whitish  blue.  But,  as  the  light 
continued  to  act,  the  cloud  became  coarser  and  whiter, 
particularly  at  its  centre,  where  it  at  length  ceased  to  dis« 
charge  polarized  light  in  the  direction  of  the  perpendicular, 
while  it  continued  to  do  so  at  both  ends. 

But  the  cloud  which  had  thus  ceased  to  polarize  the 
light  emitted  normally,  showed  vivid  selenite  colors  when 
looked  at  obliquely,  proving  that  the  direction  of  maxi- 
mum polarization  changed  with  the  texture  of  the  cloud. 
This  point  shall  receive  further  illustration  subsequently, 

A  blue,  equally  rich  and  more  durable,  was  obtained 
by  employing  the  nitrite -of -butyl  vapor  in  a  still  more  at- 
tenuated condition.  The  instance  here  cited  is  represen- 
tative. In  all  cases,  and  with  all  substances,  the  cloud 
formed  at  the  commencement,  when  the  precipitated  par- 
ticles are  sufficiently  fine,  is  blue,  and  it  can  be  made  to 
display  a  color  rivalling  that  of  the  purest  Italian  sky.  In 
all  cases,  moreover,  this  fine  blue  cloud  polarizes  perfectly 
the  beam  which  illuminates  it,  the  direction  of  polarization 
enclosing  an  angle  of  90°  with  the  axis  of  the  illuminating 
beam. 

It  is  exceedingly  interesting  to  observe  both  the  perfec- 
tion and  the  decay  of  this  polarization.  For  ten  or  fifteen 
minutes  after  its  first  appearance  the  light  from  a  vividly 
illuminated  actinic  cloud,  looked  at  perpendicularly,  is  ab- 
solutely quenched  by  a  NicoFs  prism  with  its  longer  diag- 
onal vertical.  But  as  the  sky-blue  is  gradually  rendered 
impure  by  the  growth  of  the  particles — ^in  other  words,  as 
real  clouds  begin  to  be  formed — ^the  polarization  begins  to 
decay,  a  portion  of  the  light  passing  through  the  prism 
in  all  its  positions.     It  is  worthy  of  note,  that,  for  some 


ARTIFICIAL   SKY  138 

time  after  the  cessation  of  perfect  polarization,  the  residual 
light  which  passes,  when  the  Nicol  is  in  its  position  of 
minimum,  transmission,  is  of  a  gorgeous  blue,  the  whiter 
light  of  the  cloud  being  extinguished.^  When  the  cloud 
texture  has  become  sufficiently  coarse  to  approximate  to 
that  of  ordinary  clouds,  the  rotation  of  the  Nicol  ceases 
to  have  any  sensible  effect  on  the  quantity  of  light  dis- 
charged normally. 

The  perfection  of  the  polarization,  in  a  direction  per- 
pendicular to  the  illuminating  beam,  is  also  illustrated  by 
the  following  experiment:  A  Nicol's  prism,  large  enough 
to  embrace  the  entire  beam  of  the  electric  lamp,  was  placed 
between  the  lamp  and  the  experimental  tube.  A  few  bub- 
bles of  air,  carried  through  the  liquid  nitrite  of  butyl, 
were  introduced  into  the  tube,  and  they  were  followed  by 
about  three  inches  (measured  by  the  mercurial  gauge)  of 
air  which  had  passed  through  aqueous  hydrochloric  acid. 
Sending  the  polarized  beam  through  the  tube,  I  placed 
myself  in  front  of  it,  my  eye  being  on  a  level  with  its 
axis,  my  assistant  occupying  a  similar  position  behind  the 
tube.  The  short  diagonal  of  the  large  Nicol  was  in  the 
first  instance  vertical,  the  plane  of  vibration  of  the  emer- 
gent beam  being  therefore  also  vertical.  As  the  light 
continued  to  act,  a  superb  blue  cloud,  visible  to  both  my 
assistant  and  myself,  was  slowly  formed.  But  this  cloud, 
so  deep  and  rich  when  looked  at  from  the  positions  men- 
tioned, utterly  disappeared  when  looked  at  vertically  doion- 
wardy  or  vertically  upward.  Reflection  from  the  cloud  was 
not  possible   in   these  directions.     When   the  large  Nicol 


*  This  shows  that  particles  too  large  to  polarize  the  blue,  polarize  perfectly- 
light  of  lower  refrangibility. 


124  FRAGMENTS   OF  SCIENCE 

was  slowly  turned  round  its  axis,  the  eye  of  tlie  observer 
being  on  the  level  of  the  beam,  and  the  line  of  vision 
perpendicular  to  it,  entire  extinction  of  the  light  emitted 
horizontally  occurred  when  the  longer  diagonal  of  the 
large  Nicol  was  vertical.  But  now  a  vivid  blue  cloud 
was  seen  when  looked  at  downward  or  upward.  This 
truly  fine  experiment,  which  I  contemplated  making  on 
my  own  account,  was  first  definitely  suggested  by  a 
remark  in  a  letter  addressed  to  me  by  Professor  Stokes. 
As  regards  the  polarization  of  skylight,  the  greatest 
stumbling-block  has  hitherto  been,  that,  in  accordance 
with  the  law  of  Brewster,  which  makes  the  index  of  re- 
fraction the  tangent  of  the  polarizing  angle,  the  reflection 
which  produces  perfect  polarization  would  require  to  be 
made  in  air  upon  air;  and  indeed  this  led  many  of  our 
most  eminent  men,  Brewster  himself  among  the  number,  to 
entertain  the  idea  of  aerial  molecular  reflection.*  I  have, 
however,  operated  upon  substances  of  widely  different  re- 


'  "The  cause  of  the  polarization  is  evidently  a  reflection  of  the  sun's  light 
upon  something.  The  question  is  on  what  ?  Were  the  angle  of  maximum 
polarization  16°,  we  should  look  to  water  or  ice  as  the  reflecting  body,  how- 
ever inconceivable  the  existence  in  a  cloudless  atmosphere  and  a  hot  summer's 
day  of  unevaporated  molecules  (particles  ?)  of  water.  But  though  we  were 
once  of  this  opinion,  careful  observation  has  satisfied  us  that  90°,  or  there- 
about, is  the  correct  angle,  and  that  therefore  whatever  be  the  body  on  which 
the  light  has  been  reflected,  if  polarized  by  a  single  reflection,  the  polarizing 
angle  must  be  45"*,  and  the  index  of  refraction,  which  is  the  tangent  of  that 
angle,  unity;  in  other  words,  the  reflection  would  require  to  be  made  in  air 
upon  airl"     (Sir  John  Herschel,  ''Meteorology,*'  par.  233.) 

Any  particles,  if  small  enough,  will  produce  both  the  color  and  the  polar- 
iaation  of  the  sky.  But  is  the  existence  of  small  water-particles  on  a  hot 
summer's  day  in  the  higher  regions  of  our  atmosphere  inconceivable  ?  It  is  to 
be  remembered  that  the  oxygen  and  nitrogen  of  the  air  behave  as  a  vacuum 
to  radiant  heat,  the  exceedingly  attenuated  vapor  of  the  higher  atmosphere 
being  therefore  in  practical  contact  with  the  cold  of  space. 


ARTIFICIAL   SKY  125 

fractive  indices,  and  therefore  of  very  different  polarizing 
angles  as  ordinarily  defined,  but  the  polarization  of  the 
beam,  by  the  incipient  cloud,  has  thus  far  proved  itself  to 
be  absolutely  independent  of  the  polarizing  angle.  The 
law  of  Brewster  does  not  apply  to  matter  in  this  condi- 
tion, and  it  rests  with  the  undulatory  theory  to  explain 
why.  Whenever  the  precipitated  particles  are  sufficiently 
fine,  no  matter  what  the  substance  forming  the  particles 
may  be,  the  direction  of  maximum  polarization  is  at  right 
angles  to  the  illuminating  beam,  the  polarizing  angle  for 
matter  in  this  condition  being  invariably  45°. 

Suppose  our  atmosphere  surrounded  by  an  envelope 
impervious  to  light,  but  with  an  aperture  on  the  sunward- 
side  through  which  a  parallel  beam  of  solar  light  could 
enter  and  traverse  the  atmosphere.  Surrounded  by  air 
not  directly  illuminated,  the  track  of  such  a  beam  would 
resemble  that  of  the  parallel  beam  of  the  electric  lamp 
through  an  incipient  cloud.  The  sunbeam  would  be  blue, 
and  it  would  discharge  laterally  light  in  precisely  the  same 
condition  as  that  discharged  by  the  incipient  cloud.  In 
fact,  the  azure  revealed  by  such  a  beam  would  be  to  all 
intents  and  purposes  that  which  I  have  called  a  "blue 
cloud."  Conversely  our  "blue  cloud"  is,  to  all  intents 
and  purposes,   an  artificial  shy.^ 

*  The  opinion  of  Sir  John  Herschel,  connecting  the  polarization  and  the 
blue  color  of  the  sky,  is  verified  by  the  foregoing  results.  *'The  more  the  sub- 
ject [the  polarization  of  skylight]  is  considered,'*  writes  this  eminent  philoso- 
pher, "the  more  it  will  be  found  beset  with  difficulties,  and  its  explanation 
when  arrived  at  will  probably  be  found  to  carry  u^ith  it  that  of  the  blue  color 
of  the  sky  itself,  and  of  the  great  quantity  of  light  it  actually  does  send  down 
to  U8.'*  ""We  may  observe,  too,"  he  adds,  *'that  it  is  only  where  the  purity  of 
the  sky  is  most  absolute  that  the  polarization  is  developed  in  its  highest  degree, 
and  that  where  there  is  the  slightest  perceptible  tendency  to  cirrus  it  is  mate- 
rially impaired.'*     This  applies  word  for  word  to  our  **incipient  clouds.*' 


126  FRAGMENTS   OF  SCIENCE 

But,  as  regards  tlie  polarization  of  the  sky,  we  know 
that  not  only  is  the  direction  of  maximum  polarization  at 
right  angles  to  the  track  of  the  solar  beams,  but  that  at 
certain  angular  distances,  probably  variable  ones,  from  the 
sun,  ''neutral  points,"  or  points  of  no  polarization,  exist, 
on  both  sides  of  which  the  planes  of  atmospheric  polar- 
ization are  at  right  angles  to  each  other.  I  have  made 
various  observations  upon  this  subject  which  are  reserved 
for  the  present;  but,  pending  the  more  complete  examina- 
tion of  the  question,  the  following  facts  bearing  upon  it 
may  be  submitted. 

The  parallel  beam  employed  in  these  experiments 
tracked  its  way  through  the  laboratory  air,  exactly  as 
sunbeams  are  seen  to  do  in  the  dusty  air  of  London. 
I  have  reason  to  believe  that  a  great  portion  of  the  mat- 
ter thus  floating  in  the  laboratory  air  consists  of  organic 
particles,  which  are  capable  of  imparting  a  perceptibly 
bluish  tint  to  the  air.  These  also  showed,  though  far 
less  vividly,  all  the  effects  of  polarization  obtained  with 
the  incipient  clouds.  The  light  discharged  laterally  from 
the  track  of  the  illuminating  beam  was  polarized,  though 
not  perfectly,  the  direction  of  maximum  polarization  being 
at  right  angles  to  the  beam.  At  all  points  of  the  beam, 
moreover,  throughout  its  entire  length,  the  light  emitted 
normally  was  in  the  same  state  of  polarization.  Keeping 
the  positions  of  the  Nicol  and  the  selenite  constant,  the 
same  colors  were  observed  throughout  the  entire  beam, 
when  the  line  of  vision  was  perpendicular  to  its   length. 

The  horizontal  column  of  air,  thus  illuminated,  was 
eighteen  feet  long,  and  could  therefore  be  looked  at  very 
obliquely.  I  placed  myself  near  the  end  of  the  beam,  as 
it  issued  from  the  electric  lamp,  and,  looking  through  the 


ARTIFICIAL   SKY  127 

Nicol  and  selenite  more  and  more  obliquely  at  the  beam, 
observed  the  colors  fading  until  they  disappeared.  Aug- 
menting the  obliquity  the  colors  appeared  once  more,  but 
they  were  now  complementary  to  the  former  ones. 

Hence  this  beam,  like  the  sky,  exhibited  a  neutral 
point,  on  opposite  sides  of  which  the  light  was  polarized 
in  planes  at  right  angles  to  each  other. 

Thinking  that  the  action  observed  in  the  laboratory 
might  be  caused,  in  some  way,  by  the  vaporous  fames 
diffused  in  its  air,  I  had  the  light  removed  to  a  room  at 
the  top  of  the  Eoyal  Institution.  The  track  of  the  beam 
was  seen  very  finely  in  the  air  of  this  room,  a  length  of 
fourteen  or  fifteen  feet  being  attainable.  This  beam  ex- 
hibited all  the  effects  observed  with  the  beam  in  the  labo- 
ratory. Even  the  uncondensed  electric  light  falling  on 
the  floating  matter  showed,  though  faintly,  the  effects 
of  polarization. 

When  the  air  was  so  sifted  as  to  entirely  remove  the 
visible  floating  matter,  it  no  longer  exerted  any  sensible 
action  upon  the  light,  but  behaved  like  a  vacuum.  The 
light  is  scattered  and  polarized  by  particles,  not  by  mole- 
cules or  atoms. 

By  operating  upon  the  fiunes  of  chloride  of  ammo- 
nium, the  smoke  of  brown  paper,  and  tobacco-smoke,  I  had 
varied  and  confirmed  in  many  ways  those  experiments  on 
neutral  points,  when  my  attention  was  drawn  by  Sir 
Charles  Wheatstone  to  an  important  observation  com- 
municated to  the  Paris  Academy  in  1860  by  Professor 
Govi,  of  Turin.*  M.  Govi  had  been  led  to  examine  a 
beam  of  light  sent  through  a  room  in  which  were  succes- 

'  "Oomptes  Rendus,*'  tome  li.  pp.  360  and  669. 


128  FRAGMENTS   OF  SCIENCE 

sively  diffused  the  smoke  of  incense,  and  tobacco -smoke. 
His  first  brief  commnnication  stated  the  fact  of  polariza- 
tion by  such  smoke;  but  in  his  second  communication  he 
announced  the  discovery  of  a  neutral  point  in  the  beam, 
at  the  opposite  sides  of  which  the  light  was  polarized  in 
planes  at  right  angles  to  each  other. 

But  unlike  my  observations  on  the  laboratory  air,  and 
unlike  the  action  of  the  sky,  the  direction  of  maximum 
polarization  in  M.  Govi's  experiment  enclosed  a  very- 
small  angle  with  the  axis  of  the  illuminating  beam.  The 
question  was  left  in  this  condition,  and  I  am  not  aware 
that  M.  Govi  or  any  other  investigator  has  pursued  it 
further. 

I  had  noticed,  as  before  stated,  that  as  the  clouds 
formed  in  the  experimental  tube  became  denser,  the  po- 
larization of  the  light  discharged  at  right  angles  to  the 
beam  became  weaker,  the  direction  of  maximum  polariza- 
tion becoming  oblique  to  the  beam.  Experiments  on  the 
fumes  of  chloride  of  ammonium  gave  me  also  reason  to 
suspect  that  the  position  of  the  neutral  point  was  not 
constant,  but  that  it  varied  with  the  density  of  the  illumi- 
nated fumes. 

The  examination  of  these  questions  led  to  the  follow- 
ing new  and  remarkable  results:  The  laboratory  being 
well  filled  with  the  fumes  of  incense,  and  sufficient  time 
being  allowed  for  their  uniform  diffusion,  the  electric 
beam  was  sent  through  the  smoke.  From  the  track  of 
the  beam  polarized  light  was  discharged;  but  the  direc- 
tion of  maximum  polarization,  instead  of  being  perpen- 
dicular, now  enclosed  an  angle  of  only  12**  or  13°  with  the 
axis  of  the  beam. 

A  neutral  point,  with  complementary  effects  at  oppo- 


AMTIFICIAL   SKY  129 

site  sides  of  it,  was  also  exhibited  by  the  beam.  The 
angle  enclosed  by  the  axis  of  the  beam,  and  a  line  drawn 
from  the  neutral  point  to  the  observer's  eye,  measured  in 
the  first  instance  %&", 

The  windows  of  the  laboratory  were  now  opened  for 
some  minutes,  a  portion  of  the  incense-smoke  being  per- 
mitted to  escape.  On  again  darkening  the  room  and  turn- 
ing on  the  light,  the  line  of  vision  to  the  neutral  point 
was  found  to  enclose,  with  the  axis  of  the  beam,  an  angle 
of  63°. 

The  windows  were  again  opened  for  a  few  minutes, 
more  of  the  smoke  being  permitted  to  escape.  Measured 
as  before,  the  angle  referred  to  was  found  to  be  54°. 

This  process  was  repeated  three  additional  times;  the 
neutral  point  was  found  to  recede  lower  and  lower  down 
the  beam,  the  angle  between  a  line  drawn  from  the  eye 
to  the  neutral  point  and  the  axis  of  the  beam  falling  suc- 
cessively from  54°  to  49°,  43°  and  83°. 

The  distances,  roughly  measured,  of  the  neutral  point 
from  the  lamp,  corresponding  to  the  foregoing  series  of 
observations,  were  these: 

1st  observatfon 2  feet    2  inches 

2d  "  2    *•      6      " 

3d  •*  .         .         .        .         .  2    "    10      ** 

4th         "  .....  3    "      2      " 

5th         "  3    "      7      " 

6th         "  4    *•      6      *' 

At  the  end  of  this  series  of  experiments  the  direction 
of  maximum  polarization  had  again  become  normal  to  the 
beam. 

The  laboratory  was  next  filled  with  the  fumes  of  gun. 
powder.    In  five  successive  experiments,  corresponding  to 


130  FRAGMENTS   OF  SCIENCE 

five  different  densities  of  the  gunpowder- smoke,  the  angles 
enclosed  between  the  line  of  vision  to  the  neutral  point 
and  the  axis  of  the  beam  were  6S%  60°,  47°,  42°,  and  88° 
respectively. 

After  the  clouds  of  gunpowder  had  cleared  away,  the 
laboratory  was  filled  with  the  fumes  of  common  resin, 
rendered  so  dense  as  to  be  very  irritating  to  the  lungs. 
The  direction  of  maximum  polarization  enclosed,  in  this 
case,  an  angle  of  12°,  or  thereabout,  with  the  axis  of  the 
beam.  Looked  at,  as  in  the  former  instances,  from  a  posi- 
tion near  the  electric  lamp,  no  neutral  point  was  observed 
throughout  the  entire  extent  of  the  beam. 

When  this  beam  was  looked  at  normally  through  the 
selenite  and  Nicol,  the  ring- system,  though  not  brilliant, 
was  distinct.  Keeping  the  eye  upon  the  plate  of  selenite, 
and  the  line  of  vision  perpendicular,  the  windows  were 
opened,  the  blinds  remaining  undrawn.  The  resinous 
fumes  slowly  diminished,  and  as  they  did  so  the  ring- 
system  became  paler.  It  finally  disappeared.  Continuing 
to  look  in  the  same  direction,  the  rings  revived,  but  now 
the  colors  were  complementary  to  the  former  ones.  The 
neutral  point  had  passed  me  in  its  motion  down  the  beamy 
consequent  upon  the  attenuation  of  the  fumes  of  resin. 

"With  the  fumes  of  chloride  of  ammonium  substantially 
the  same  results  were  obtained.  Sufiicient,  however,  has 
been  here  stated  to  illustrate  the  variability  of  the  position 
of  the  neutral  point.  * 

By  a   puff  of  tobacco- smoke,   or   of   condensed  steam, 


*  Brewster  haa  proved  the  variability  of  the  position  of  the  neutral  point  for 
skylight  with  the  sun's  altitude,  a  result  obviously  connected  with  the  foregoing 
experiments. 


ARTIFICIAL   SKY  131 

blown  into  the  illuminated  beam,  the  brilliancy  of  the 
selenite  colors  may  be  greatly  enhanced.  But  with  differ- 
ent clouds  two  different  effects  are  produced.  Let  the 
ring-system  observed  in  the  common  air  be  brought  to  its 
maximum  strength,  and  then  let  an  attenuated  cloud  of 
chloride  of  ammonium  be  thrown  into  the  beam  at  the 
point  looked  at ;  the  ring-system  flashes  out  with  aug- 
mented brilliancy,  but  the  character  of  the  polarization 
remains  unchanged.  This  is  also  the  case  when  phos- 
phorus, or  sulphur,  is  burned  underneath  the  beam,  so 
as  to  cause  the  fine  particles  of  phosphorus  or  of  sulphur 
to  rise  into  the  light.  With  the  sulphur-fiunes  the  bril- 
liancy of  the  colors  is  exceedingly  intensified;  but  in  none 
of  these  cases  is  there  any  change  in  the  character  of  the 
polarization. 

But  when  a  puff  of  the  fumes  of  hydrochloric  acid, 
hydriodic  acid,  or  nitric  acid  is  thrown  into  the  beam, 
there  is  a  complete  reversal  of  the  selenite  tints.  Each  of 
these  clouds  twists  the  plane  of  polarization  90°,  causing 
the  centre  of  the  ring-system  to  change  from  black  to 
white,  and  the  rings  themselves  to  emit  their  comple- 
mentary colors.' 

Almost  all  liquids  have  motes  in  them  sufficiently 
numerous  to  polarize  sensibly  the  light,  and  very  beau- 
tiful effects  may  be  obtained  by  simple  artificial  devices. 
When,  for  example,  a  cell  of  distilled  water  is  placed  in 
front  of  the  electric  lamp,  and  a  thin  slice  of  the  beam  is 


'  Sir  John  Herschel  suggested  to  me  that  this  change  of  the  polarization  from 
positive  to  negative  may  indicate  a  change  from  polarization  by  reflection  to 
polarization  by  refraction.  This  thought  repeatedly  occurred  to  me  while  look- 
ing at  the  effects;  but  it  will  require  much  following  up  before  it  emerges  into 
clearness. 


132  FRAGMENTS   OF  SCIENCE 

permitted  to  pass  through  it,  scarcely  any  polarized  light 
is  discharged,  and  scarcely  any  color  produced  with  a 
plate  of  selenite.  But  if  a  bit  of  soap  be  agitated  in  the 
water  above  the  beam,  the  moment  the  infinitesimal  par* 
tides  reach  the  light  the  liquid  sends  forth  laterally  al- 
most perfectly  polarized  light;  and  if  the  selenite  be  em- 
ployed, vivid  colors  flash  into  existence.  A  still  more 
brilliant  result  is  obtained  with  mastic  dissolved  in  a  great 
excess  of  alcohol. 

The  selenite  rings,  in  fact,  constitute  an  extremely 
delicate  test  as  to  the  collective  quantity  of  individually 
invisible  particles  in  a  liquid.  Commencing  with  distilled 
water,  for  example,  a  thick  slice  of  light  is  necessary  to 
make  the  polarization  of  its  suspended  particles  sensible. 
A  much  thinner  slice  suffices  for  common  water;  while, 
with  Brtlcke's  precipitated  mastic,  a  slice  too  thin  to  pro- 
duce any  sensible  effect  with  most  other  liquids,  suffices 
to  bring  out  vividly  the  selenite  colors. 

§  8.    The  Skt  op  the  Alps 

The  vision  of  an  object  always  implies  a  differential 
action  on  the  retina  of  the  observer.  The  object  is  dis- 
tinguished from  surrounding  space  by  its  excess  or  defect 
of  light  in  relation  to  that  space.  By  altering  the  illu- 
mination, either  of  the  object  itself  or  of  its  environment, 
we  alter  the  appearance  of  the  object.  Take  the  case  of 
clouds  floating  in  the  atmosphere  with  patches  of  blue 
between  them.  Anything  that  changes  the  illumination 
of  either  alters  the  appearance  of  both,  that  appearance 
depending,  as  stated,  upon  differential  action.  Kow  the 
light  of  the  sky,  being  polarized,  may,  as  the  reader  of 


THE  SKY  OF  THE  ALPS  133 

the  foregoing  pages  knows,  be  in  great  part  quenched  by 
a  Nicors  prism,  while  the  light  of  a  common  cloud,  being 
unpolarized,  cannot  be  thus  extinguished.  Hence  the  pos- 
sibility of  very  remarkable  variations,  not  only  in  the 
aspect  of  the  firmament,  which  is  really  changed,  but  also 
in  the  aspect  of  the  clouds,  which  have  that  firmament 
as  a  background.  It  is  possible,  for  example,  to  choose 
clouds  of  such  a  depth  of  shade  that  when  the  Nicol 
quenches  the  light  behind  them,  they  shall  vanish,  being 
indistinguishable  from  the  residual  dull  tint  which  out- 
lives the  extinction  of  the  brilliancy  of  the  sky.  A  cloud 
less  deeply  shaded,  but  still  deep  enough,  when  viewed 
with  the  naked  eye,  to  appear  dark  on  a  bright  ground, 
is  suddenly  changed  to  a  white  cloud  on  a  dark  ground 
by  the  quenching  of  the  light  behind  it  When  a  reddish 
cloud  at  sunset  chances  to  float  in  the  region  of  maximum 
polarization,  the  quenching  of  the  surrounding  light  causes 
it  to  flash  with  a  brighter  crimson.  Last  Easter  eve  the 
Dartmoor  sky,  which  had  just  been  cleansed  by  a  snow- 
storm, wore  a  very  wild  appearance.  Eound  the  horizon 
it  was  of  steely  brilliancy,  while  reddish  cumuli  and  cirri 
floated  southward.  When  the  sky  was  quenched  behind 
them  these  floating  masses  seemed  like  dull  embers  sud- 
denly blown  upon;   they  brightened  like  a  fire. 

In  the  Alps  we  have  the  most  magnificent  examples 
of  crimson  clouds  and  snows,  so  that  the  effects  just  re- 
ferred to  may  be  here  studied  under  the  best  possible 
conditions.  On  August  23,  1869,  the  evening  Alpen-glow 
was  very  fine,  though  it  did  not  reach  its  maximum  depth 
and  splendor.  The  side  of  the  Weisshom  seen  from  the 
Bel  Alp,  being  turned  from  the  sun,  was  tinted  mauve; 
but  I  wished  to  observe  one  of  the  rose -colored  buttresses 


134  FRAGMENTS   OF  SCIENCE 

of  the  mountain.  Such  a  one  was  visible  from  a  point  a 
few  hundred  feet  above  the  hotel.  The  Matterhorn  also, 
though  for  the  most  part  in  shade,  had  a  crimson  projec- 
tion, while  a  deep  ruddy  red  lingered  along  its  western 
shoulder.  Four  distinct  peaks  and  buttresses  of  the  Dom, 
in  addition  to  its  dominant  head — all  covered  with  pure 
snow — were  reddened  by  the  light  of  sunset.  The  shoul- 
der of  the  Alphubel  was  similarly  colored,  while  the  great 
mass  of  the  Fletschorn  was  all  aglow,  and  so  was  the 
snowy  spine  of  the  Monte  Leone. 

Looking  at  the  Weisshorn  through  the  Nicol,  the  glow 
of  its  protuberance  was  strong  or  weak  according  to  the 
position  of  the  prism.  The  summit  also  underwent  strik- 
ing changes.  In  one  position  of  the  prism  it  exhibited  a 
pale  white  against  a  dark  background;  in  the  rectangular 
position  it  was  a  dark  mauve  against  a  light  background. 
The  red  of  the  Matterhorn  changed  in  a  similar  manner; 
but  the  whole  mountain  also  passed  through  wonderful 
changes  of  definition.  The  air  at  the  time  was  filled  with 
a  silvery  haze,  in  which  the  Matterhorn  almost  disap- 
peared. This  could  be  wholly  quenched  by  the  Nicol, 
and  then  the  mountain  sprang  forth  with  astonishing  so- 
lidity and  detachment  from  the  surrounding  air.  The 
changes  of  the  Dom  were  still  more  wonderful.  A  vast 
amount  of  light  could  be  removed  from  the  sky  behind 
it,  for  it  occupied  4ihe  position  of  maximum  polarization. 
By  a  little  practice  with  the  Nicol  it  was  easy  to  render 
the  extinction  of  the  light,  or  its  restoration,  almost  in- 
stantaneous. When  the  sky  was  quenched,  the  four  minor 
peaks  and  buttresses,  and  the  summit  of  the  Dom,  to- 
gether with  the  shoulder  of  the  Alphubel,  glowed  as  if 
set  suddenly  on  fire.     This  was  immediately  dimmed  by 


THE   SKY   OF   THE  ALPS  136 

turning  the  Nicol  througli  an  angle  of  90°.  It  was  not 
the  stoppage  of  the  light  of  the  sky  behind  the  mountains 
alone  which  produced  this  startling  effect;  the  air  be- 
tween them  and  me  was  highly  opalescent,  and  the 
quenching  of  this  intermediate  glare  augmented  remark- 
ably the  distinctness  of  the  mountains. 

On  the  morning  of  August  24  similar  effects  were 
finely  shown.  At  10  A.M.  all  three  mountains,  the  Dom, 
the  Matterhorn,  and  the  Weisshorn,  were  powerfully 
affected  by  the  Nicol.  But  in  this  instance  also,  the  line 
drawn  to  the  Dom  being  very  nearly  perpendicular  to  the 
solar  beams,  the  effects  on  this  mountain  were  most  strik- 
ing. The  gray  summit  of  the  Matterhorn,  at  the  same 
time,  could  scarcely  be  distinguished  from  the  opalescent 
haze  around  it;  but  when  the  Nicol  quenched  the  haze, 
the  summit  became  instantly  isolated,  and  stood  out  in 
bold  definition.  It  is  to  be  remembered  that  in  the  pro* 
duction  of  these  effects  the  only  things  changed  are  the 
sky  behind,  and  the  luminous  haze  in  front  of  the  moun- 
tains; that  these  are  changed  because  the  light  emitted 
from  the  sky  and  from  the  haze  is  plane  polarized  light, 
and  that  the  light  from  the  snows  and  from  the  moun- 
tains, being  sensibly  unpolarized,  is  not  directly  affected 
by  the  Nicol.  It  will  also  be  understood  that  it  is  not 
the  interposition  of  the  haze  as  an  opaque  body  that  ren- 
ders the  mountains  indistinct,  but  that  it  is  the  light  of 
the  haze  which  dims  and  bewilders  the  eye,  and  thus 
weakens  the  definition  of  objects  seen  through  it. 

These  results  have  a  direct  bearing  upon  what  artists 
call  '* aerial  perspective."  As  we  look  from  the  eummit 
of  Mont  Blanc,  or  from  a  lower  elevation,  at  the  serried 
crowd  of  peaks,    especially  if   the    mountains   be  darkly 


136  FRAGMENTS   OF  SCIENCE 

colored — covered  with  pines,  for  example — every  peak  and 
ridge  is  separated  from  the  mountains  behind  it  by  a  thin 
blue  haze  which  renders  the  relations  of  the  mountains 
as  to  distance  unmistakable.  When  this  haze  is  regarded 
through  the  Nicol  perpendicular  to  the  sun's  rays,  it  is 
in  many  cases  wholly  quenched,  because  the  light  which 
it  emits  in  this  direction  is  wholly  polarized.  When  this 
happens,  aerial  perspective  is  abolished,  and  mountains 
very  differently  distant  appear  to  rise  in  the  same  vertical 
plane.  Close  to  the  Bel  Alp,  for  instance,  is  the  gorge 
of  the  Massa,  and  beyond  the  gorge  is  a  high  ridge  dark- 
ened by  pines.  This  ridge  may  be  projected  upon  the 
dark  slopes  at  the  opposite  side  of  the  Ehone  valley,  and 
between  both  we  have  the  blue  haze  referred  to,  throwing 
the  distant  mountains  far  away.  But  at  certain  hours  of 
the  day  the  haze  may  be  quenched,  and  then  the  Massa 
ridge  and  the  mountains  beyond  the  Ehone  seem  almost 
equally  distant  from  the  eye.  The  one  appears,  as  it 
were,  a  vertical  continuation  of  the  other.  The  haze 
varies  with  the  temperature  and  humidity  of  the  atmos- 
phere. At  certain  times  and  places  it  is  almost  as  blue 
as  the  sky  itself;  but  to  see  its  color,  the  attention  must 
be  withdrawn  from  the  mountains  and  from  the  trees 
which  cover  them.  In  point  of  fact,  the  haze  is  a  piece 
of  more  or  less  perfect  sky;  it  is  produced  in  the  same 
manner,  and  is  subject  to  the  same  laws,  as  the  firmament 
itself.     We  live  in  the  sky,  not  under  it. 

These  points  were  further  elucidated  by  the  deport- 
ment of  the  selenite  plate,  with  which  the  readers  of  the 
foregoing  pages  are  so  well  acquainted.  On  some  of 
the  sunny  days  of  August  the  haze  in  the  valley  of  the 
Ehone,  as  looked  at  from  the  Bel  Alp,  was  very  remark- 


THE   SKY  OF   THE  ALPS  137 

able.  Toward  evening  the  sky  above  the  mountains  op- 
posite to  my  place  of  observation  yielded  a  series  of  the 
most  splendidly-colored  iris-rings;  but  on  lowering  the  sel- 
enite  until  it  had  the  darkness  of  the  pines  at  the  oppo- 
site side  of  the  Rhone  valley,  instead  of  the  darkness  of 
space,  as  a  background,  the  colors  were  not  much  dimin- 
ished in  brilliancy.  I  should  estimate  the  distance  across 
the  valley,  as  the  crow  flies,  to  the  opposite  mountain, 
at  nine  miles;  so  that  a  body  of  air  of  this  thickness  can, 
under  favorable  circumstances,  produce  chromatic  effects 
of  polarization  almost  as  vivid  as  those  produced  by  the 
sky  itself. 

Again:  the  light  of  a  landscape,  as  of  most  other  things, 
consists  of  two  parts;  the  one,  coming  purely  from  super- 
ficial reflection,  is  always  of  the  same  color  as  the  light 
which  falls  upon  the  landscape;  the  other  part  reaches  us 
from  a  certain  depth  within  the  objects  which  compose  the 
landscape,  and  it  is  this  portion  of  the  total  light  which 
gives  these  objects  their  distinctive  colors.  The  white 
light  of  the  sun  enters  all  substances  to  a  certain  depth, 
and  is  partly  ejected  by  internal  reflection;  each  distinct 
substance  absorbing  and  reflecting  the  light,  in  accordance 
with  the  laws  of  its  own  molecular  constitution.  Thus  the 
solar  light  is  sifted  by  the  landscape,  which  appears  in 
such  colors  and  variations  of  color  as,  after  the  sifting 
process,  reach  the  observer's  eye.  Thus  the  bright  green 
of  grass,  or  the  darker  color  of  the  pine,  never  comes  to 
us  alone,  but  is  always  mingled  with  an  amount  of  light 
derived  from  superficial  reflection.  A  certain  hard  bril- 
liancy is  conferred  upon  the  woods  and  meadows  by  this 
superficially-reflected  light.  Under  certain  circumstances, 
it  may  be  quenched  by  a  Nicol's  prism,  and  we  then  ob- 


138  FRAGMENTS   OF  SCIENCE 

tain  the  true  color  of  the  grass  and  foliage.  Trees  and 
meadows,  thus  regarded,  exhibit  a  richness  and  softness 
of  tint  which  they  never  show  as  long  as  the  superficial 
light  is  permitted  to  mingle  with  the  true  interior  emis- 
sion. The  needles  of  the  pines  show  this  effect  very  well, 
large-leaved  trees  still  better;  while  a  glimmering  field  of 
maize  exhibits  the  most  extraordinary  variations  when 
looked   at  through   the   rotating   Nicol. 

Thoughts  and  questions  like  those  here  referred  to  took 
me,  in  August,  1869,  to  the  top  of  the  Aletschhorn.  The 
effects  described  in  the  foregoing  paragraphs  were,  for  the 
most  part,  reproduced  on  the  summit  of  the  mountain.  I 
scanned  the  whole  of  the  sky  with  my  Nicol.  Both  alone, 
and  in  conjunction  with  the  selenite,  it  pronounced  the 
perpendicular  to  the  solar  beams  to  be  the  direction  of 
maximum  polarization.  But  at  no  portion  of  the  firma- 
ment was  the  polarization  complete.  The  artificial  sky 
produced  in  the  experiments  recorded  in  the  preceding 
pages  could,  in  this  respect,  be  rendered  far  more  per- 
fect than  the  natural  one;  while  the  gorgeous  ** residual 
blue,"  which  makes  its  appearance  when  the  polarization 
of  the  artificial  sky  ceases  to  be  perfect,  was  strongly  con- 
trasted with  the  lack-lustre  hue  which,  in  the  case  of  the 
firmament,  outlived  the  extinction  of  the  brilliancy.  With 
certain  substances,  however,  artificially  treated,  this  dull 
residue  may  also  be  obtained. 

All  along  the  arc,  from  the  Matterhorn  to  Mont  Blanc, 
the  light  of  the  sky  immediately  above  the  mountains  was 
powerfully  acted  upon  by  the  Nicol.  In  some  cases  the 
variations  of  intensity  were  astonishing.  I  have  already 
said  that  a  little  practice  enables  the  observer  to  shift  the 
Nicol  from  one  position  to  another  so  rapidly  as  to  render 


THE  SKY  OF  THE  ALPS  139 

the  alternative  extinction  and  restoration  of  the  light  im- 
mediate. When  this  was  done  along  the  arc  to  which  I 
have  referred,  the  alternations  of  light  and  darkness  re- 
sembled the  play  of  sheet  lightning  behind  the  mountains. 
There  was  an  element  of  awe  connected  with  the  sudden- 
ness with  which  the  mighty  masses,  ranged  along  the  line 
referred  to,  changed  their  aspect  and  definition  under  the 
operation  of  the  prism. 


pn  the  last  edition  of  the  "Fragments  of  Science,"  an  essay  on  "Dust  and 
Disease"  followed  here;  but,  as  almost  all  my  writings  on  the  *'Germ  Theory" 
are  now  collected  in  a  single  volume  entitled  * 'Essays  on  the  Floating  Matter  of 
the  Air,"  **Du8t  and  Disease"  no  longer  appears  in  the  "Fragments."  In  its 
place  I  venture  to  introduce  a  short  article  written  early  last  year  for  an  impor- 
tant American  magazine.] 


THE  SKY  * 

INVITED   to   write   for   the    ** Forum'*    an   article   that 
would  have  brought  me  face  to  face  with  *' problems 
of  life  and  mind,"  for  which  I  was  at  the  moment 
unprepared,  and  unwilling  to  decline  a  request  so  cour- 
teously made,  I  offered,  if  the  editor  cared  to  accept  it,  to 
send  him  a  contribution  on  the  subject  here  presented. 

I  mentioned  this  subject,  thinking  that,  in  addition  to 
its  interest  as  a  fragment  of  ** natural  knowledge,"  it  might 
permit  of  a  glance  at  the  workings  of  the  scientific  mind 
when  engaged  on  the  deeper  problems  which  come  before 
it.  In  the  house  of  Science  are  many  mansions,  occupied 
by  tenants  of  diverse  kinds.  Some  of  them  execute  with 
painstaking  fidelity  the  useful  work  of  observation,  re- 
cording from  day  to  day  the  aspects  of  Nature  or  the  indi- 
cations of  instruments  devised  to  reveal  her  ways.  Others 
there  are  who  add  to  this  capacity  for  observation  a  power 
over  the  language  of  experiment,  by  means  of  which  they 
put  questions  to  Nature,  and  receive  from  her  intelligible 
replies.  There  is,  again,  a  third  class  of  minds  that  can- 
not rest  content  with  observation  and  experiment,  whose 
love  of   causal   unity   tempts   them  perpetually  to   break 

»  From  "The  Forum,"  February,  1888. 

(140) 


THE  SKY  141 

througli  the  limitations  of  the  senses,  and  to  seek  beyond 
them  the  roots  and  reasons  of  the  phenomena  which  the 
observer  and  experimenter  record.  To  such  spirits — ad- 
venturous and  firm — we  are  indebted  for  our  deeper  knowl- 
edge of  the  methods  by  which  the  physical  universe  is 
ordered  and  ruled. 

In  his  efforts  to  cross  the  common  bourne  of  the  known 
and  the  unknown,  the  effective  force  of  the  man  of  science 
must  depend,  to  a  great  extent,  upon  his  acquired  knowl- 
edge. But  knowledge  alone  will  not  do ;  a  stored  memory 
will  not  suffice;  inspiration  must  lend  its  aid.  Scientific 
inspiration,  however,  is  usually,  if  not  always,  the  fruit 
of  long  reflection — of  patiently  ''intending  the  mind,'*  as 
Kewton  phrased  it;  and  as  Copernicus,  Newton,  and  Dar- 
win practiced  it;  until  outer  darkness  yields  a  glimmer, 
which  in  due  time  opens  out  into  perfect  intellectual  day. 
From  some  of  his  expressions  it  might  be  inferred  that 
Newton  scorned  hypotheses;  but  he  allowed  them,  never- 
theless,  an  open  avenue  to  his  own  mind.  He  propounded 
the  famous  corpuscular  theory  of  light,  illustrating  it  and 
defending  it  with  a  skill,  power,  and  fascination  which 
subsequently  won  for  it  ardent  supporters  among  the  best 
intellects  of  the  world.  This  theory,  moreover,  was 
weighted  with  a  supplementary  hypothesis,  which  as- 
cribed to  the  luminiferous  molecules  **fits  of  easy  refleo* 
tion  and  transmission,*'  in  virtue  of  which  they  were  some- 
tirnes  repelled  from  the  surfaces  of  bodies  and  sometimes 
permitted  to  pass  through.  Newton  may  have  scorned  the 
levity  with  which  hypotheses  are  sometimes  framed;  but 
he  lived  in  an  atmosphere  of  theory,  which  he,  like  all 
profound  scientific  thinkers,  found  to  be  the  very  breath 
of  his  intellectual  liie. 


142  FRAGMENTS   OF  SCIENCE 

The  theorist  takes  his  conceptions  from  the  world  of 
fact,  and  refines  and  alters  them  to  suit  his  needs.  The 
sensation  of  sound  was  known  to  be  produced  by  aerial 
waves  impinging  on  the  auditory  nerve.  Air  being  a 
thing  that  could  be  felt,  and  its  vibrations,  by  suitable 
treatment,  made  manifest  to  the  eye,  there  was  here  a 
physical  basis  for  the  ** scientific  imagination'*  to  build 
upon.  Both  Hooke  and  Huyghens  built  upon  it  with 
effect.  By  the  illustrious  astronomer  last  named  the  con- 
ception of  waves  was  definitely  transplanted  from  its  ter- 
restrial birthplace  to  a  universal  medium  whose  undula- 
tions could  only  be  intellectually  discerned.  Huyghens 
did  not  establish  the  undulatory  theory,  but  he  took  the 
first  firm  step  toward  establishing  it.  Laying  this  theory 
at  the  root  of  the  phenomena  of  light,  he  went  a  good  way 
toward  showing  that  these  phenomena  are  the  necessary 
outgrowth  of  the  conception. 

By  analysis  and  synthesis  Newton  proved  the  white 
light  of  the  sun  to  be  a  skein  of  many  colors.  The  cause 
of  color  was  a  question  which  immediately  occupied  his 
thoughts;  and  here,  as  in  pther  cases,  he  freely  resorted 
to  hypothesis.  He  saw,  with  his  mind's  eye,  his  luminif- 
erous  corpuscles  crossing  the  bodily  eye,  and  imparting 
successive  shocks  to  the  retina  behind.  To  differences  of 
*' bigness"  in  the  light-awakening  molecules  Newton  as- 
cribed the  different  color-sensations.  In  the  undulatory 
theory  we  are  also  confronted  with  the  question  of  color; 
and  here  again,  to  inform  and  guide  us,  we  have  the  anal- 
ogy of  sound.  Aerial  waves  of  different  lengths,  or  peri- 
ods, produce  notes  of  different  pitch;  and  to  differences 
of  wave-length  in  that  mysterious  medium,  the  all-pervad- 
ing ether,  differences  of  color  are  ascribed.      Hooke  had 


THE  SKt  143 

already  discoursed  of  **a  very  quick  motion  that  causes 
light,  as  well  as  a  more  robust  that  causes  heat.**  New- 
ton had  ascribed  the  sensation  of  red  to  the  shock  of  his 
grossest,  and  that  of  violet  to  the  shock  of  his  finest,  lumi- 
niferous  projectiles.  Defining  the  one,  and  displacing  the 
other  of  these  notions,  the  wave-theory  affirms  red  to  be 
produced  by  the  largest,  and  violet  by  the  smallest  waves 
of  the  visible  spectrum.  The  theory  of  undulation  had  to 
encounter  that  fierce  struggle  for  existence  which  all  great 
changes  of  doctrine,  scientific  or  otherwise,  have  had  to 
endure.  Mighty  intellects,  following  the  mightiest  of  them 
all,  were  arrayed  against  it.  But  the  more  it  was  discussed 
the  more  it  grew  in  strength  and  favor,  until  it  finally  sup- 
planted its  formidable  rival.  No  competent  scientific  man 
at  the  present  day  accepts  the  theory  of  emission,  or  refuses 
to  accept  the  theory  of  undulation. 

Boyle  and  Hooke  had  been  fruitful  experimenters  on 
those  beautiful  iridescences  known  as  the  "colors  of  thin 
plates.**  The  rich  hues  of  the  thin-blown  soap-bubble,  of 
oil  floating  on  water,  and  of  the  thin  layer  of  oxide  on 
molten  lead,  are  familiar  illustrations  of  these  iris  colors. 
Hooke  showed  that  all  transparent  films,  if  only  thin 
enough,  displayed  such  colors;  and  he  proved  that  the 
particular  color  displayed  depended  upon* the  thickness  of 
the  film.  Passing  from  solid  and  liquid  films  to  films 
of  air,  he  says:  "Take  two  small  pieces  of  ground  and 
polished  looking-glass  plate,  each  about  the  bigness  of  a 
shilling;  take  these  two  dry,  and  with  your  forefingers 
and  thumbs  press  them  very  hard  and  close  together,  and 
you  shall  find  that,  when  they  approach  each  other  very 
near,  there  will  appear  several  irises  or  colored  lines." 
Newton,  bent  on  knowing  the  exact  relation  between  the 


144  FRAGMENTS   OF  SCIENCE 

thickness  of  the  film  and  the  color  it  produced,  varied 
Hooke's  experiment.  Taking  two  pieces  of  glass,  the  one 
plane  and  the  other  very  slightly  curved,  and  pressing 
both  together,  he  obtained  a  film  of  air  of  gradually  in- 
creasing thickness  from  the  place  of  contact  outward. 
As  he  expected,  he  found  the  place  of  contact  surrounded 
by  a  series  of  colored  circles,  still  known  all  over  the 
world  as  *' Newton's  rings."  The  colors  of  his  first  cir- 
cle, which  immediately  surrounded  a  black  central  spot, 
Newton  called  "colors  of  the  first  order";  the  colors  of 
the  second  circle,  "colors  of  the  second  order,"  and  so 
on.  With  unrivalled  penetration  and  apparent  success, 
he  applied  his  theory  of  "fits"  to  the  explanation  of  the 
"rings."  Here,  however,  the  only  immortal  parts  of  his 
labors  are  his  facts  and  measurements;  his  theory  has 
disappeared.  It  was  reserved  for  the  illustrious  Thomas 
Young,  a  man  of  intellectual  calibre  resembling  that  of 
Newton  himself,  to  prove  that  the  rings  were  produced 
by  the  mutual  action — ^in  technical  phrase,  "interference" 
—of  the  light- waves  reflected  at  the  two  surfaces  of  the 
film  of  air  enclosed  between  the  plane  and  convex  glasses. 
The  colors  of  thin  plates  were  "residual  colors" — surviv' 
als  of  the  white  light  after  the  ravages  of  interference. 
Young  soon  translated  the  theory  of  "fits"  into  that  of 
"waves";  the  measurements  pertaining  to  the  former  be- 
ing so  accurate  as  to  render  them  immediately  available 
for  the  purposes  of  the  latter. 

It  is  here  that  Newton's  researches  and  opinions  touch 
the  subject  of  this  article.  The  color  nearest  to  the  black 
spot,  in  the  experiment  above  described,  was  a  faint  blue 
— "blue  of  the  first  order" — corresponding  to  the  film  of 
air  when  thinnest.     If  a  solid  or  liquid  film,  of  the  thick- 


THE   SKY  145 

ness  requisite  to  produce  this  color,  were  broken  into  bits 
and  scattered  in  the  air,  Newton  inferred  that  the  tiny 
fragments  would  display  the  blue  color.  Tantamount  to 
this,  he  considered,  was  the  action  of  minute  water-parti- 
cles in  the  incipient  stage  of  their  condensation  from  aque- 
ous vapor.  Such  particles  suspended  in  our  atmosphere 
ought,  he  supposed,  to  generate  the  serenes  t  skies.  New- 
ton does  not  appear  to  have  bestowed  much  thought  upon 
this  subject;  for  to  produce  the  particular  blue  which  he 
regarded  as  sky-blue,  thin  plates  with  parallel  surfaces 
would  be  required.  The  notion  that  cloud-particles  are 
hollow  spheres,  or  vesicles,  is  prevalent  on  the  Continent, 
but  it  never  made  any  way  among  the  scientific  men  of 
England.  De  Saussure  thought  that  he  had  actually  seen 
the  cloud- vesicles,  and  Faraday,  as  I  learned  from  himself, 
believed  that  he  had  once  confirmed  the  observation  of  the 
illustrious  Alpine  traveller.  During  my  long  acquaintance 
with  the  atmosphere  of  the  Alps  I  have  often  sought  for 
these  aqueous  bladders,  but  have  never  been  able  to  find 
them.  Clausius  once  published  a  profound  essay  on  the 
colors  of  the  sky.  The  assumption  of  small  water  drops, 
he  proved,  would  lead  to  optical  consequences  entirely 
at  variance  with  facts.  For  a  time,  therefore,  he  closed 
with  the  idea  of  vesicles,  and  endeavored  to  deduce  from 
them  the  blue  of  the  firmament  and  the  momiDg  and 
evening  red. 

It  is  not,  however,  necessary  to  invoke  the  blue  of  the 
first  order  to  explain  the  color  of  the  sky;  nor  is  it  neces- 
sary to  impose  upon  condensing  vapor  the  difficult,  if  not 
impossible,  task  of  forming  bladders,  when  it  passes  into 
the  liquid  condition.  Let  us  examine  the  subject.  Eau' 
de-  Cologne  is  prepared  by  dissolving  aromatic  gums  or  res- 

SCIENCE — V — 7 


146  FRAGMENTS  OF  SCIENCE 

ins  in  alcoliol.  Dropped  into  water,  the  scented  jfiquid 
immediately  produces  a  white  cloudiness,  due  to  the  pre- 
cipitation of  the  substances  previously  held  in  solution. 
The  solid  particles  are,  however,  comparatively  gross ;  but, 
by  diminishing  the  quantity  of  the  dissolved  gum,  the  pre- 
cipitate may  be  made  to  consist  of  extremely  minute  parti- 
cles. Briicke,  for  example,  dissolved  gum-mastic,  in  cer- 
tain proportions,  in  alcohol,  and  carefully  dropping  his 
solution  into  a  beaker  of  water,  kept  briskly  stirred,  he 
was  able  to  reduce  the  precipitate  to  an  extremely  fine 
Btate  of  division.  The  particles  of  mastic  can  by  no  means 
be  imagined  as  forming  bladders.  Still,  against  a  dark 
ground — black  velvet,  for  example — the  water  that  con- 
tains them  shows  a  distinctly  blue  color.  The  bluish  color 
of  many  liquids  is  produced  in  a  similar  manner.  Thin 
milk  is  an  example.  Blue  eyes  are  also  said  to  be  simply 
turbid  media.  The  rocks  over  which  glaciers  pass  are 
finely  ground  and  pulverized  by  the  ice,  or  the  stony 
emery  imbedded  in  it;  and  the  river  which  issues  from 
the  snout  of  every  glacier  is  laden  with  suspended  matter. 
When  such  glacier  water  is  placed  in  a  tall  glass  jar,  and 
the  heavier  particles  are  permitted  to  subside,  the  liquid 
column,  when  viewed  against  a  dark  background,  has  a 
decidedly  bluish  tinge.  The  exceptional  blueness  of  the 
Ijake  of  Geneva,  which  is  fed  with  glacier  water,  may  be 
due,  in  part,  to  particles  small  enough  to  remain  suspended 
long  after  their  larger  and  heavier  companions  have  sunk 
to  the  bottom  of  the  lake. 

We  need  not,  however,  resort  to  water  for  the  produc- 
tion of  the  color.  We  can  liberate,  in  air,  particles  of  a 
size  capable  of  producing  a  blue  as  deep  and  pure  as  the 
azure  of  the  firmament.      In  fact,  artificial  skies  may  be 


THE  SKY  147 

thus  generated,  which  prove  their  brotherhood  with  the 
natural  sky  by  exhibiting  all  its  phenomena.  There  are 
certain  chemical  compounds — aggregates  of  molecules — the 
constituent  atoms  of  which  are  readily  shaken  asunder  by 
the  impact  of  special  waves  of  light.  Probably,  if  not  cer- 
tainly, the  atoms  and  the  waves  are  so  related  to  each 
other,  as  regards  vibrating  period,  that  the  wave- motion 
can  accumulate  until  it  becomes  disruptive.  A  great  num- 
ber of  substances  might  be  mentioned  whose  vapors,  when 
mixed  with  air  and  subjected  to  the  action  of  a  solar  or 
an  electric  beam,  are  thus  decomposed,  the  products  of 
decomposition  hanging  as  liquid  or  solid  particles  in  the 
beam  which  generates  them.  And  here  I  must  appeal  to 
the  inner  vision  already  spoken  of.  Eemembering  the  dif- 
ferent sizes  of  the  waves  of  light,  it  is  not  difficult  to  see 
that  our  minute  particles  are  larger  with  respect  to  some 
waves  than  to  others.  In  the  case  of  water,  for  example, 
a  pebble  will  intercept  and  reflect  a  larger  fractional. part 
of  a  ripple  than  of  a  larger  wave.  We  have  now  to  im- 
agine light- undulations  of  different  dimensions,  but  all  ex- 
ceedingly minute,  passing  through  air  laden  with  extremely 
small  particles.  It  is  plain  that  such  particles,  though  scat- 
tering portions  of  all  the  waves,  will  exert  their  most  con- 
spicuous action  upon  the  smallest  ones;  and  that  the  color- 
sensation  answering  to  the  smallest  waves — in  other  words, 
the  color  line — will  be  predominant  in  the  scattered  light. 
This  harmonizes  perfectly  with  what  we  observe  in  the 
firmament.  The  sky  is  blue,  but  the  blue  is  not  pure. 
On  looking  at  the  sky  through  a  spectroscope,  we  observe 
all  the  colors  of  the  spectrum;  blue  is  merely  the  predom- 
inant color.  By  means  of  our  artificial  skies  we  can  take, 
as  it  were,  the  firmament  in  our  hands  and  examine  it  at 


148  FRAGMENTS   OF  SCIENCE 

our  leisure.  Like  the  natural  sky,  the  artificial  one  shows 
all  the  colors  of  the  spectrum,  but  blue  in  excess.  Mixing 
very  small  quantities  of  vapor  with  air,  and  bringing  the 
decomposing  luminous  beam  into  action,  we  produce  parti- 
cles too  small  to  shed  any  sensible  light,  but  which  may, 
and  doubtless  do,  exert  an  action  on  the  ultra-violet  waves 
of  the  spectrum.  We  can  watch  these  particles,  or  rather 
the  space  they  occupy,  till  they  grow  to  a  size  able  to 
yield  the  firmamental  azure.  As  the  particles  grow  larger 
under  the  continued  action  of  the  light,  the  azure  becomes 
less  deep;  while  later  on  a  milkiness,  such  as  we  often  ob- 
serve in  nature,  takes  the  place  of  the  purer  blue.  Finally 
the  particles  become  large  enough  to  reflect  all  the  light- 
waves, and  then  the  suspended  * 'actinic  cloud"  diifuses 
white  light. 

It  must  occur  to  the  reader  that  even  in  the  absence 
of  definite  clouds  there  are  considerable  variations  in  the 
hue  of  the  firmament.  Everybody  knows,  moreover,  that 
as  the  sky  bends  toward  the  horizon,  the  purer  blue  is 
impaired.  To  measure  the  intensity  of  the  color  De  Saus- 
sure  invented  a  cyanometer,  and  Humboldt  has  given  us 
a  mathematical  formula  to  express  the  diminution  of  the 
blue,  in  arcs  drawn  east  and  west  from  the  zenith  down- 
ward. This  diminution  is  a  natural  consequence  of  the 
predominance  of  coarser  particles  in  the  lower  regions  of 
the  atmosphere.  Were  the  particles  which  produce  the 
purer  celestial  vault  all  swept  away,  we  should,  unless 
helped  by  what  has  been  called  "cosmic  dust,*'  look  into 
the  blackness  of  celestial  space.  And  were  the  whole 
atmosphere  abolished  along  with  its  suspended  matter,  we 
should  have  the  ''blackness"  spangled  with  steady  stars; 
for  the  twinkling  of  the  stars  is  caused  by  our  atmos- 


THE   SKY  14d 

pliere.  Now,  the  higher  we  ascend,  the  more  do  we  leave 
behind  us  the  particles  which  scatter  the  light;  the  nearer, 
in  fact,  do  we  approach  to  that  vision  of  celestial  space 
mentioned  a  moment  ago.  Viewed,  therefore,  from  the 
loftiest  Alpine  summits,  the  firmamental  blue  is  darker 
than  it  is  ever  observed  to  be  from  the  plains. 

It  is  thus  shown  that  by  the  scattering  action  of  mi- 
nute particles  the  blue  of  the  sky  can  be  produced;  but 
there  is  yet  more  to  be  said  upon  the  subject.  Let  the 
natural  sky  be  looked  at  on  a  fine  day  through  a  piece  of 
transparent  Iceland  spar  cut  into  the  form  known  as  a 
Nicol  prism.  It  may  be  well  to  begin  by  looking  through 
the  prism  at  a  snow  slope,  or  a  white  wall.  Turning  the 
prism  round  its  axis,  the  light  coming  from  these  objects 
does  not  undergo  any  sensible  change.  But  when  the 
prism  is  directed  toward  the  sky  the  great  probability  is 
that,  on  turning  it,  variations  in  the  amount  of  light 
reaching  the  eye  will  be  observed.  Testing  various  por- 
tions of  the  sky  with  due  diligence,  we  at  length  discover 
one  particular  direction  where  the  difference  of  illumina- 
tion becomes  a  maximum.  Here  the  Nicol,  in  one  posi- 
tion, seems  to  offer  no  impediment  to  the  passage  of  the 
skylight;  while,  when  turned  through  an  arc  of  ninety 
degrees  from  this  position,  the  light  is  almost  entirely 
quenched.  We  soon  discern  that  the  particular  line  of 
vision  in  which  this  maximum  difference  is  observed  is 
perpendicular  to  the  direction  of  the  solar  rays.  The 
Nicol  acts  thus  upon  skylight  because  that  light  is  polar- 
ized, while  the  light  from  the  white  wall  or  the  white 
snow,  being  unpolarized,  is  not  affected  by  the  rotation 
of  the  prism. 

In  the  case  of  our  manufactured  sky  not  only  is  the 


150  FRAGMENTS   OF  SCIENCE 

azure  of  the  firmament  reproduced,  but  these  phenomena 
of  polarization  are  observed  even  more  perfectly  than  in 
the  natural  sky.  When  the  air-space  from  which  our  best 
artificial  azure  is  emitted  is  examined  with  the  Nicol 
prism,  the  blue  light  is  found  to  be  completely  polarized 
at  right  angles  to  the  illuminating  beam.  The  artificial 
sky  may,  in  fact,  be  employed  as  a  second  Nicol,  between 
which  and  a  prism  held  in  the  hand  many  of  the  beautiful 
chromatic  phenomena  observed  in  an  ordinary  polariscope 
may  be  reproduced. 

Let  us  now  complete  our  thesis  by  following  the  larger 
light-waves,  which  have  been  able  to  pass  among  the 
aerial  particles  with  comparatively  little  fractional  loss. 
Without  going  beyond  inferential  considerations,  we  can 
state  what  must  occur.  The  action  of  the  particles  upon 
the  solar  light  increases  with  the  atmospheric  distances 
traversed  by  the  sun's  rays.  The  lower  the  sun,  there- 
fore, the  greater  the  action.  The  shorter  waves  of  the 
spectrum  being  more  and  more  withdrawn,  the  tendency 
is  to  give  the  longer  waves  an  enhanced  predominance 
in  the  transmitted  light.  The  tendency,  in  other  words, 
of  this  light,  as  the  rays  traverse  ever- increasing  dis- 
tances, is  more  and  more  toward  red.  This,  I  say,  might 
be  stated  as  an  inference,  but  it  is  borne  out  in  the  most 
impressive  manner  by  facts.  When  the  Alpine  sun  is 
aetting,  or,  better  still,  some  time  after  he  has  set,  leav- 
ing the  limbs  and  shoulders  of  the  mountains  in  shadow, 
while  their  snowy  crests  are  bathed  by  the  retreating 
light,  the  snow  glows  with  a  beauty  and  solemnity  hardly 
equalled  by  any  other  natural  phenomenon.  So,  also, 
when  first  illumined  by  the  rays  of  the  unrisen  sun,  the 
mountain   heads,   under  favorable  atmospheric  conditions, 


THE  SKY  151 

shine  like  rubies.  And  all  this  splendor  is  evoked  by  tbe 
simple  mechanism  of  minute  particles,  themselves  without 
color,  suspended  in  the  air.  Those  who  referred  the  ex- 
traordinary succession  of  atmospheric  glows,  witnessed 
some  years  ago,  to  a  vast  and  violent  discharge  of  vol- 
canic ashes,  were  dealing  with  "a  true  cause."  The  fine 
floating  residue  of  such  ashes  would,  undoubtedly,  be 
able  to  produce  the  effects  ascribed  to  it.  Still,  the  mech- 
anism necessary  to  produce  the  morning  and  the  evening 
red,  though  of  variable  efficiency,  is  always  present  in  the 
atmosphere.  I  have  seen  displays,  equal  in  magnificence 
to  the  finest  of  those  above  referred  to,  when  there  was 
no  special  volcanic  outburst  to  which  they  could  be  re- 
ferred. It  was  the  long-continued  repetition  of  the  glows 
which  rendered  the  volcanic  theory  highly  probable. 


VI 

VOYAGE  TO   ALGERIA    TO    OBSERVE   THE   ECLIPSE 

1870 

THE  opening  of  the  Eclipse  Expedition  was  not  pro- 
pitious. Portsmouth,  on  Monday,  December  6, 
1870,  was  swathed  by  fog,  which  was  intensified 
by  smoke,  and  traversed  by  a  drizzle  of  fine  rain.  At 
six  P.M.,  I  was  on  board  the  "Urgent."  On  Tuesday 
morning  the  weather  was  too  thick  to  permit  of  the  ship's 
being  swung  and  her  compasses  calibrated.  The  admiral 
of  the  port,  a  man  of  very  noble  presence,  came  on  board. 
Under  his  stimulus  the  energy  which  the  weather  had 
damped  appeared  to  become  more  active,  and  soon  after 
his  departure  we  steamed  down  to  Spithead.  Here  the 
fog  had  so  far  lightened  as  to  enable  the  officers  to  swing 
the  ship. 

At  three  p.m.  on  Tuesday,  December  6,  we  got  away, 
gliding  successively  past  Whitecliff  Bay,  Bembridge,  San- 
down,  Shanklin,  Yentnor,  and  St.  Catherine's  Lighthouse. 
On  Wednesday  morning  we  sighted  the  Isle  of  Ushant, 
on  the  French  side  of  the  Channel.  The  northern  end  of 
the  island  has  been  fretted  by  the  waves  into  detached 
tower- like  masses  of  rock  of  very  remarkable  appearance. 
In  the  Channel  the  sea  was  green,  and  opposite  Ushant 
it  was  a  brighter  green.  On  Wednesday  evening  we  com- 
mitted ourselves  to  the  Bay  of  Biscay.  The  roll  of  the 
Atlantic  was  full,  but  not  violent.  There  had  been 
(152) 


VOYAGE   TO    ALGERIA  153 

scarcely  a  gleam  of  sunsliine  throughout  the  day,  but  the 
cloud-forms  were  fine,  and  their  apparent  solidity  impres- 
sive. On  Thursday  morning  the  green  of  the  sea  was 
displaced  by  a  deep  indigo  blue.  The  whole  of  Thurs- 
day we  steamed  across  the  bay.  We  had  little  blue  sky, 
but  the  clouds  were  again  grand  and  varied — cirrus,  stra- 
tus, cumulus,  and  nimbus,  we  had  them  all.  Dusky  hair- 
like trails  were  sometimes  dropped  from  the  distant  clouds 
to  the  sea.  These  were  falling  showers,  and  they  some- 
times occupied  the  whole  horizon,  while  we  steamed 
across  the  rainless  circle  which  was  thus  surrounded. 
Sometimes  we  plunged  into  the  rain,  and  once  or  twice, 
by  slightly  changing  our  course,  avoided  a  heavy  shower. 
From  time  to  time  perfect  rainbows  spanned  the  heavens 
from  side  to  side.  At  times  a  bow  would  appear  in  frag- 
ments, showing  the  keystone  of  the  arch  midway  in  air, 
and  its  two  buttresses  on  the  horizon.  In  all  cases  the 
light  of  the  bow  could  be  quenched  by  a  Nicol's  prism, 
with  its  long  diagonal  tangent  to  the  arc.  Sometimes 
gleaming  patches  of  the  firmament  were  seen  amid  the 
clouds.  When  viewed  in  the  proper  direction,  the  gleam 
could  be  quenched  by  a  Nicol's  prism,  a  dark  aperture 
being  thus  opened  into  stellar  space. 

At  sunset  on  Thursday  the  denser  clouds  were  fiercely 
fringed,  while  through  the  lighter  ones  seemed  to  issue 
the  glow  of  a  conflagration.  On  Friday  morning  we 
sighted  Cape  Finisterre — the  extreme  end  of  the  arc 
which  sweeps  from  Ushant  round  the  Bay  of  Biscay. 
Calm  spaces  of  blue,  in  which  floated  quietly  scraps  of 
cumuli,  were  behind  us,  but  in  front  of  us  was  a  horizon 
of  portentous  darkness.  It  continued  thus  threatening 
throughout  the  day.     Toward  evening  the  wind  strength- 


154  FRAGMENTS   OF  SCIENCE 

ened  to  a  gale,  and  at  dinner  it  was  difficult  to  preserve 
tlie  plates  and  dishes  from  destruction.  Our  thinned 
company  hinted  that  the  rolling  had  other  consequences. 
It  was  very  wild  when  we  went  to  bed.  I  slumbered 
and  slept,  but  after  some  time  was  rendered  anxiously 
conscious  that  my  body  had  become  a  kind  of  projectile, 
with  the  ship's  side  for  a  target.  I  gripped  the  edge  of 
my  berth  to  save  myself  from  being  thrown  out.  Out- 
side, I  could  hear  somebody  say  that  he  had  been  thrown 
from  his  berth,  and  sent  spinning  to  the  other  side  of  the 
saloon.  The  screw  labored  violently  amid  the  lurching; 
it  incessantly  quitted  the  water,  and,  twirling  in  the  air, 
rattled  against  its  bearings,  causing  the  ship  to  shudder 
from  stem  to  stern.  At  times  the  waves  struck  us,  not 
with  the  soft  impact  which  might  be  expected  from  a 
liquid,  but  with  the  sudden  solid  shock  of  battering-rams. 
"No  man  knows  the  force  of  water,"  said  one  of  the  offi- 
cers, ''until  he  has  experienced  a  storm  at  sea."  These 
blows  followed  each  other  at  quicker  intervals,  the  screw 
rattling  after  each  of  them,  until,  finally,  the  delivery  of 
a  heavier  stroke  than  ordinary  seemed  to  reduce  the  sa- 
loon to  chaos.  Furniture  crashed,  glasses  rang,  and 
alarmed  inquiries  immediately  followed.  Amid  the  noises 
I  heard  one  note  of  forced  laughter;  it  sounded  very 
ghastly.  Men  tramped  through  the  saloon,  and  busy 
voices  were  heard  aft,  as  if  something  there  had  gone 
wrong. 

I  rose,  and  not  without  difficulty  got  into  my  clothes. 
In  the  after-cabin,  under  the  superintendence  of  the  able 
and  energetic  navigating  lieutenant,  Mr.  Brown,  a  group 
of  blue- jackets  were  working  at  the  ^tiller- ropes.  These 
had  become  loose,   and  the  helm  refused  to  answer  the 


VOYAGE    TO    ALGERIA  155 

wheel  High  moral  lessons  might  be  gained  on  ship- 
board, by  observing  what  steadfast  adherence  to  an  object 
can  accomplish,  and  what  large  effects  are  heaped  np  by 
the  addition  of  infinitesimals.  The  tiller-rope,  as  the 
blue- jackets  strained  in  concert,  seemed  hardly  to  move; 
still  it  did  move  a  little,  until  finally,  by  timing  the  pull 
to  the  lurching  of  the  ship,  the  mastery  of  the  rudder 
was  obtained.  I  had  previously  gone  on  deck.  Bound 
the  saloon- door  were  a  few  members  of  the  eclipse  party, 
who  seemed  in  no  mood  for  scientific  observation.  Nor 
did  I ;  but  I  wished  to  see  the  storm.  I  climbed  the  steps 
to  the  poop,  exchanged  a  word  with  Captain  Toynbee, 
the  only  member  of  the  party  to  be  seen  on  the  poop, 
and  by  his  direction  made  toward  a  cleat  not  far  from 
the  wheel.*  Round  it  I  coiled  my  arms.  With  the  ex- 
ception of  the  men  at  the  wheel,  who  stood  as  silent  as 
corpses,  I  was  alone. 

I  had  seen  grandeur  elsewhere,  but  this  was  a  new 
form  of  grandeur  to  me.  The  "Urgent"  is  long  and  nar- 
row, and  during  our  expedition  she  lacked  the  steadying 
influence  of  sufficient  ballast.  She  was  for  a  time  prac- 
tically rudderless,  and  lay  in  the  trough  of  the  sea.  I 
could  see  the  long  ridges,  with  some  hundreds  of  feet  be- 
tween their  crests,  rolling  upon  the  ship  perfectly  parallel 
to  her  sides.  As  they  approached,  they  so  grew  upon  the 
eye  as  to  render  the  expression  "mountains  high"  intel- 
ligible. At  all  events,  there  was  no  mistaking  their  me- 
chanical might,  as  they  took  the  ship  upon  their  shoulders 
and  swung  her  like  a  pendulum.  The  deck  sloped  some- 
times at  an  angle  which  I  estimated  at  over  forty-five  de- 

*  The  cleat  is  a  J-sliaped  mass  of  metal  employed  for  the  fastening  of  ropeg. 


156  FRAGMENTS   OF  SCIENCE 

grees ;  wanting  my  previous  Alpine  practice,  I  should  have 
felt  less  confidence  in  my  grip  of  the  cleat.  Here  and 
there  the  long  rollers  were  tossed  by  interference  into 
heaps  of  greater  height.  The  wind  caught  their  crests, 
and  scattered  them  over  the  sea,  the  whole  surface  of 
which  was  seething  white.  The  aspect  of  the  clouds  was 
a  fit  accompaniment  to  the  fury  of  the  ocean.  The  moon 
was  almost  full — at  times  concealed,  at  times  revealed,  as 
the  scud  flew  wildly  over  it.  These  things  appealed  to 
the  eye,  while  the  ear  was  filled  by  the  groaning  of  the 
screw  and  the  whistle  and  boom  of  the  storm. 

Nor  was  the  outward  agitation  the  only  object  of  inter- 
est to  me.  I  was  at  once  subject  and  object  to  myself, 
and  watched  with  intense  interest  the  workings  of  my 
own  mind.  The  "Urgent"  is  an  elderly  ship.  She  had 
been  built,  I  was  told,  by  a  contracting  firm  for  some  for- 
eign Government,  and  had  been  diverted  from  her  first 
purpose  when  converted  into  a  troop- ship.  She  had  been 
for  some  time  out  of  work,  and  I  had  heard  that  one  of 
her  boilers,  at  least,  needed  repair.  Our  scanty  but  excel- 
lent crew,  moreover,  did  not  belong  to  the  "Urgent,"  but 
had  been  gathered  from  other  ships.  Our  three  lieutenants 
were  also  volunteers.  All  this  passed  swiftly  through  my 
mind  as  the  steamer  shook  under  the  blows  of  the  waves, 
and  I  thought  that  probably  no  one  on  board  could  say 
how  much  of  this  thumping  and  straining  the  "Urgent" 
would  be  able  to  bear.  This  uncertainty  caused  me  to 
look  steadily  at  the  worst,  and  I  tried  to  strengthen  myself 
in  the  face  of  it. 

But  at  length  the  helm  laid  hold  of  the  water,  and  the 
ship  was  got  gradually  round  to  face  the  waves.  The  roll- 
ing diminished,   a  certain   amount  of   pitching   taking  its 


VOYAGE    TO    ALGERIA  157 

place.  Our  speed  had  fallen  from  eleven  knots  to  two. 
I  went  again  to  bed.  After  a  space  of  calm,  when  we 
seemed  crossing  the  vortex  of  a  storm,  heavy  tossing  re- 
commenced. I  was  afraid  to  allow  myself  to  fall  asleep, 
as  my  berth  was  high,  and  to  be  pitched  out  of  it  might 
be  attended  with  bruises,  if  not  with  fractures.  From  Fri- 
day at  noon  to  Saturday  at  noon  we  accomplished  sixty- 
six  miles,  or  an  average  of  less  than  three  miles  an  hour. 
I  overheard  the  sailors  talking  about  this  storm.  The 
"Urgent,"  according  to  those  that  knew  her,  had  never 
previously  experienced  anything  like  it.* 

All  through  Saturday  the  wind,  though  somewhat  so- 
bered, blew  dead  against  us.  The  atmospheric  effects 
were  exceedingly  fine.  The  cumuli  resembled  mountains 
in  shape,  and  their  peaked  summits  shone  as  white  as 
Alpine  snows.  At  one  place  this  resemblance  was  greatly 
strengthened  by  a  vast  area  of  cloud,  uniformly  illumi- 
nated, and  lying  like  a  n^v^  below  the  peaks.  From  it 
fell  a  kind  of  cloud-river  strikingly  like  a  glacier.  The 
horizon  at  sunset  was  remarkable — spaces  of  brilliant  green 
between  clouds  of  fiery  red.  Eainbows  had  been  frequent 
throughout  the  day,  and  at  night  a  perfectly  continuous 
lunar  bow  spanned  the  heavens  from  side  to  side.  Its 
colors  were  feeble ;  but,  contrasted  with  the  black  ground 
against  which  it  rested,  its  luminousness  was  extraor- 
dinary. 

Sunday  morning  found  us  opposite  to  Lisbon,  and  at 
midnight  we  rounded  Cape  St.  Vincent,  where  the  lurch- 
ing seemed  disposed  to  recommence.     Through  the  kind- 


*  There  is,  it  will  be  seen,  a  fair  agreement  between  these  impressions  and 
those  30  vigorously  described  by  a  scientific  correspondent  of  the  "Times." 


158  FRAGMENTS   OF  SCIENCE 

ness  of  Lieutenant  Walton,  a  cot  had  been  slung  for  me. 
It  hung  between  a  tiller- wheel  and  a  flue,  and  at  one  A.M. 
I  was  roused  by  the  banging  of  the  cot  against  its  boun- 
daries. But  the  wind  was  now  behind  us,  and  we  went 
along  at  a  speed  of  eleven  knots.  We  felt  certain  of 
reaching  Cadiz  by  three.  But  a  new  lighthouse  came  in 
sight,  which  some  affirmed  to  be  Cadiz  Lighthouse,  while 
the  surrounding  houses  were  declared  to  be  those  of  Cadiz 
itself.  Out  of  deference  to  these  statements,  the  navigat- 
ing lieutenant  changed  his  course,  and  steered  for  the 
place.  A  pilot  came  on  board,  and  he  informed  us  that 
we  were  before  the  mouth  of  the  Guadalquivir,  and  that 
the  lighthouse  was  that  of  Cipiona.  Cadiz  was  still  some 
eighteen  miles  distant. 

We  steered  toward  the  city,  hoping  to  get  into  the  har- 
bor before  dark.  But  the  pilot  who  would  have  guided 
us  had  been  snapped  up  by  another  vessel,  and  we  did 
not  get  in.  We  beat  about  during  the  night,  and  in  the 
morning  found  ourselves  about  fifteen  miles  from  Cadiz. 
The  sun  rose  behind  the  city,  and  we  steered  straight  into 
the  light.  The  three- towered  cathedral  stood  in  the  midst, 
round  which  swarmed  apparently  a  multitude  of  chimney- 
stacks.  A  nearer  approach  showed  the  chimneys  to  be 
small  turrets.  A  pilot  was  taken  on  board;  for  there  is 
a  dangerous  shoal  in  the  harbor.  The  appearance  of  the 
town  as  the  sun  shone  upon  its  white  and  lofty  walls  was 
singularly  beautiful.  We  cast  anchor;  some  officials  ar- 
rived and  demanded  a  clean  bill  of  health.  We  had  none. 
They  would  have  nothing  to  do  with  us;  so  the  yellow 
quarantine  flag  was  hoisted,  and  we  waited  for  permission 
to  land  the  Cadiz  party.  After  some  hours'  delay,  the 
English  consul  and  vice-consul  came  on  board,  and  with 


VOYAGE    TO    ALGERIA  159 

them,  a  Spanish  officer  ablaze  witli  gold  lace  and  decora- 
tions. Under  slight  pressure  the  requisite  permission  had 
been  granted.  We  landed  our  party,  and  in  the  afternoon 
weighed  anchor.  Thanks  to  the  kindness  of  our  excellent 
paymaster,  I  was  here  transferred  to  a  more  roomy  berth. 
Cadiz  soon  sank  beneath  the  sea,  and  we  sighted  in 
succession  Cape  Trafalgar,  Tarifa,  and  the  revolving  light 
of  Ceuta.  The  water  was  very  calm,  and  the  moon  rose 
in  a  quiet  heaven.  She  swung  with  her  convex  surface 
downward,  the  common  boundary  between  light  and  shadow 
being  almost  horizontal.  A  pillar  of  reflected  light  shim- 
mered up  to  us  from  the  slightly  rippled  sea.  I  had  pre- 
viously noticed  the  phosphorescence  of  the  water,  but  to- 
night it  was  stronger  than  usual,  especially  among  the 
foam  at  the  bows.  A  bucket  let  down  into  the  sea 
brought  up  a  number  of  the  little  sparkling  organisms 
which  caused  the  phosphorescence.  I  caught  some  of 
them  in  my  hand.  And  here  an  appearance  was  observed 
which  was  new  to  most  of  us,  and  strikingly  beautiful  to 
all.  Standing  at  the  bow  and  looking  forward,  at  a  dis- 
tance of  forty  or  fifty  yards  from  the  ship,  a  number  of 
luminous  streamers  were  seen  rushing  toward  us.  On 
nearing  the  vessel  they  rapidly  turned,  like  a  comet 
round  its  perihelion,  placed  themselves  side  by  side,  and, 
in  parallel  trails  of  light,  kept  up  with  the  ship.  One  of 
them  placed  itself  right  in  front  of  the  bow  as  a  pioneer. 
These  comets  of  the  sea  were  joined  at  intervals  by  oth- 
ers. Sometimes  as  many  as  six  at  a  time  would  rush  at 
us,  bend  with  extraordinary  rapidity  round  a  sharp  curve, 
and  afterward  keep  us  company.  I  leaned  over  the  bow 
and  scanned  the  streamers  closely.  The  frontal  portion 
of  each  of  them  revealed  the  outline  of  a  porpoise.     The 


160  FRAGMENTS    OF   SCIENCE 

rush  of  the  creatures  tlirough  the  water  had  started  the 
phosphorescence,  every  spark  of  which  was  converted  by 
the  motion  of  the  retina  into  a  line  of  light.  Each  por- 
poise was  thus  wrapped  in  a  luminous  sheath.  The  phos- 
phorescence did  not  cease  at  the  creature's  tail,  but  was 
carried  many  porpoise-lengths  behind  it. 

To  our  right  we  had  the  African  hills,  illuminated  by 
the  moon.  Gribraltar  Eock  at  length  became  visible,  but 
the  town  remained  long  hidden  by  a  belt  of  haze,  through 
which  at  length  the  brighter  lamps  struggled.  It  was  like 
the  gradual  resolution  of  a  nebula  into  stars.  As  the  in- 
tervening depth  became  gradually  less,  the  mist  vanished 
more  and  more,  and  finally  all  the  lamps  shone  through 
it.  They  formed  a  bright  foil  to  the  sombre  mass  of  rock 
above  them.  The  sea  was  so  calm  and  the  scene  so  lovely 
that  Mr.  Huggins  and  myself  stayed  on  deck  till  near 
midnight,  when  the  ship  was  moored.  During  our  walk- 
ing to  and  fro  a  striking  enlargement  of  the  disk  of  Jupi- 
ter was  observed,  whenever  the  heated  air  of  the  funnel 
came  between  us  and  the  planet.  On  passing  away  from 
the  heated  air,  the  flat  dim  disk  would  immediately  shrink 
to  a  luminous  point.  The  effect  was  one  of  visual  per- 
sistence. The  retinal  image  of  the  planet  was  set  quiver- 
ing in  all  azimuths  by  the  streams  of  heated  air,  describing 
in  quick  succession  minute  lines  of  light,  which  summed 
themselves  to  a  disk  of  sensible  area. 

At  six  o'clock  next  morning,  the  gun  at  the  Signal 
Station  on  the  summit  of  the  rock  boomed.  At  eight 
the  band  on  board  the  "Trafalgar"  training-ship,  which 
was  in  the  harbor,  struck  up  the  national  anthem;  and  im- 
mediately afterward  a  crowd  of  mite- like  cadets  swarmed 
up  the  rigging.     After  the  removal  of  the  apparatus  be- 


VOYAGE   TO    ALGERIA  161 

longing  to  tlie  Gribraltar  party  we  went  on  shore.  Winter 
was  in  England  when  we  left,  but  here  we  had  the  warmth 
of  summer.  The  vegetation  was  luxuriant — palm-trees, 
cactuses,  and  aloes,  all  ablaze  with  scarlet  flowers.  A 
visit  to  the  Governor  was  proposed,  as  an  act  of  neces- 
sary courtesy,  and  I  accompanied  Admiral  Ommaney  and 
Mr.  Huggins  to  "the  Convent,"  or  G-overnment  House. 
We  sent  in  our  cards,  waited  for  a  time,  and  were  then 
conducted  by  an  orderly  to  his  Excellency.  He  is  a  fine 
old  man,  over  six  feet  high,  and  of  frank  military  bearing. 
He  received  us  and  conversed  with  us  in  a  very  genial 
manner.  He  took  us  to  see  his  garden,  his  palms,  his 
shaded  promenades,  and  his  orange-trees  loaded  with  fruit, 
in  all  of  which  he  took  manifest  delight.  Evidently  "the 
hero  of  Kars"  had  fallen  upon  quarters  after  his  own 
heart.  He  appeared  full  of  good  nature,  and  engaged 
us  on  the  spot  to  dine  with  him  that  day. 

We  sought  the  town-major  for  a  pass  to  visit  the  lines. 
While  awaiting  his  arrival  I  purchased  a  stock  of  white 
glass  bottles,  with  a  view  to  experiments  on  the  color  of 
the  sea.  Mr.  Huggins  and  myself,  who  wished  to  see  the 
rock,  were  taken  by  Captain  Salmon d  to  the  library,  where 
a  model  of  Gibraltar  is  kept,  and  where  we  had  a  useful 
preliminary  lesson.  At  the  library  we  met  Colonel  Ma- 
berly,  a  courteous  and  kindly  man,  who  gave  us  good 
advice  regarding  our  excursion.  He  sent  an  orderly  with 
us  to  the  entrance  of  the  lines.  The  orderly  handed  us 
over  to  an  intelligent  Irishman,  who  was  directed  to  show 
us  everything  that  we  desired  to  see,  and  to  hide  nothing 
from  us.  We  took  the  "upper  line,"  traversed  the  gal- 
leries hewn  through  the  limestone;  looked  through  the 
embrasures,  which  opened  like  doors  in  the  precipice,  to- 


162  FRAGMENTS   OF  SCIENCE 

ward  tlie  hills  of  Spain;  reached  St.  George's  hall,  and 
went  still  higher,  emerging  on  the  summit  of  one  of  the 
noblest  clifife  I  have  ever  seen. 

Beyond  were  the  Spanish  lines,  marked  by  a  line  of 
white  sentry-boxes;  nearer  were  the  English  lines,  less 
conspicuously  indicated;  and  between  both  was  the  neu- 
tral ground.  Behind  the  Spanish  lines  rose  the  conical 
hill  called  the  Queen  of  Spain's  Chair.  The  general  as- 
pect of  the  mainland  from  the  rock  is  bold  and  rugged. 
Doubling  back  from  the  galleries,  we  struck  upward 
toward  the  crest,  reached  the  Signal  Station,  where  we 
indulged  in  **  shandy -gaff"  and  bread  and  cheese.  Thence 
to  O'Hara's  Tower,  the  highest  point  of  the  rock.  It 
was  built  by  a  former  governor,  who,  forgetful  of  the 
laws  of  terrestrial  curvature,  thought  he  might  look  from 
the  tower  into  the  port  of  Cadiz.  The  tower  is  riven,  and 
it  may  be  climbed  along  the  edges  of  the  crack.  We  got 
to  the  top  of  it;  thence  descended  the  curious  Mediter- 
ranean Stair — a  zigzag,  mostly  of  steps  down  a  steeply 
falling  slope,  amid  palmetto  brush,  aloes,  and  prickly 
pear. 

Passing  over  the  Windmill  Hill,  we  were  joined  at  the 
**Governor's  Cottage"  by  a  car,  and  drove  afterward  to 
the  lighthouse  at  Europa  Point.  The  tower  was  built, 
I  believe,  by  Queen  Adelaide,  and  it  contains  a  fine  diop- 
tric apparatus  of  the  first  order,  constructed  by  Messrs. 
Chance,  of  Birmingham.  At  the  appointed  hour  we  were 
at  the  Convent.  During  dinner  the  same  genial  traits 
which  appeared  in  the  morning  were  still  more  conspic- 
uous. The  freshness  of  the  Governor's  nature  showed 
itself  best  when  he  spoke  of  his  old  antagonist  in  arms, 
Mouravieff.     Chivalry  in  war  is  consistent  with  its  stern 


VOYAGE    TO   ALGERIA  163 

prosecution.  These  two  men  were  chivalrous,  and  after 
striking  the  last  blow  became  friends  forever.  Our  kind 
and  courteous  reception  at  Gibraltar  is  a  thing  to  be  re- 
membered with  pleasure. 

On  December  15  we  committed  ourselves  to  the  Medi- 
terranean. The  views  of  Gibraltar  with  which  we  are 
most  acquainted  represent  it  as  a  huge  ridge;  but  its  as- 
pect, end  on,  both  from  the  Spanish  lines  and  from  the 
other  side,  is  truly  noble.  There  is  a  sloping  bank  of 
sand  at  the  back  of  the  rock,  which  I  was  disposed  to 
regard  simply  as  the  debris  of  the  limestone.  I  wished 
to  let  myself  down  upon  it,  but  had  not  the  time.  My 
friend  Mr.  Busk,  however,  assures  me  that  it  is  silica, 
and  that  the  same  sand  constitutes  the  adjacent  neutral 
ground.  There  are  theories  afloat  as  to  its  having  been 
blown  from  Sahara.  The  Mediterranean  throughout  this 
first  day,  and  indeed  throughout  the  entire  voyage  to 
Oran,  was  of  a  less  deep  blue  than  the  Atlantic.  Pos- 
sibly the  quantity  of  organisms  may  have  modified  the 
color.  At  night  the  phosphoresence  was  startling,  break- 
ing suddenly  out  along  the  crests  of  the  waves  formed  by 
the  port  and  starboard  bows.  Its  strength  was  not  uni- 
form. Having  flashed  brilliantly  for  a  time,  it  would 
in  part  subside,  and  afterward  regain  its  vigor.  Several 
large  phosphorescent  masses  of  weird  appearance  also 
floated  past. 

On  the  morning  of  the  16th  we  sighted  the  fort  and 
lighthouse  of  Marsa  el  Kibir,  and  beyond  them  the  white 
walls  of  Oran  lying  in  the  bight  of  a  bay,  sheltered  by 
dominant  hills.  The  sun  was  shining  brightly;  during 
our  whole  voyage  we  had  not  had  so  fine  a  day.  The 
wisdom  which  had  led  us  to  choose    Oran  as  our  place 


164  FRAGMENTS   OF  SCIENCE 

of  observation  seemed  demonstrated.  A  rather  excitable 
pilot  came  on  board,  and  lie  guided  us  in  behind  tbe 
Mole,  which  had  suffered  much  damage  the  previous 
year  from  an  unexplained  outburst  of  waves  from  the 
Mediterranean.  Both  port  and  bow  anchors  were  cast  in 
deep  water.  With  three  huge  hawsers  the  ship's  stem 
was  made  fast  to  three  gun-pillars  fixed  in  the  Mole;  and 
here  for  a  time  the  "Urgent"  rested  from  her  labors. 

M.  Janssen,  who  had  rendered  his  name  celebrated  by 
his  observations  of  the  eclipse  in  India  in  1868,  when 
he  showed  the  solar  flames  to  be  eruptions  of  incandes- 
cent hydrogen,  was  already  encamped  in  the  open  country 
about  eight  miles  from  Oran.  On  December  2  he  had 
quitted  Paris  in  a  balloon,  with  a  strong  young  sailor  as 
his  assistant,  had  descended  near  the  mouth  of  the  Loire, 
seen  M.  Gambetta,  and  received  from  him  encouragement 
and  aid.  On  the  day  of  our  arrival  his  encampment  was 
visited  by  Mr.  Huggins,  and  the  kind  and  courteous 
Engineer  of  the  Port  drove  me  subsequently,  in  his  own 
phaeton,  to  the  place.  It  bore  the  best  repute  as  regards 
freedom  from  haze  and  fog,  and  commanded  an  open  out- 
look; but  it  was  inconvenient  for  us  on  account  of  its 
distance  from  the  ship.  The  place  next  in  repute  was 
the  railway  station,  between  two  and  three  miles  distant 
from  the  Mole.  It  was  inspected,  but,  being  enclosed, 
was  abandoned  for  an  eminence  in  an  adjacent  garden, 
the  property  of  Mr.  Hinshelwood,  a  Scotchman  who  had 
settled  some  years  previously  as  an  Esparto  merchant  in 
Oran. '  He,  in  the  most  liberal  manner,  placed  his  ground 
at    the    disposition    of    the    party.     Here    the  tents  were 


*  Esparto  is  a  kind  of  grass  now  much  used  in  the  manufacture  of  paper. 


VOYAGE    TO    ALGERIA  165 

pitched,  on  tlie  Saturday,  by  Captain  Salmond  and  liis 
intelligent  corps  of  sappers,  the  instruments  being  erected 
on  the  Monday  under  cover  of  the  tents. 

Close  to  the  railway  station  runs  a  new  loopholed  wall 
of  defence,  through  which  the  highway  passes  into  the 
open  country.  Standing  on  the  highway,  and  looking 
southward,  about  twenty  yards  to  tbe  right  is  a  small 
bastionet,  intended  to  carry  a  gun  or  two.  Its  roof  I 
thought  would  form  an  admirable  basis  for  my  telescope, 
while  the  view  of  the  surrounding  country  was  unimpeded 
in  all  directions.  The  authorities  kindly  allowed  me  the 
use  of  this  bastionet.  Two  men,  one  a  blue-jacket  named 
Elliot,  and  the  other  a  marine  named  Hill,  were  placed 
at  my  disposal  by  Lieutenant  "Walton;  and,  thus  aided, 
on  Monday  morning  I  mounted  my  telescope.  The  in- 
strument was  new  to  me,  and  some  hours  of  discipline 
were  spent  in  mastering  all  the  details  of  its  manipulation. 

Mr.  Huggins  joined  me,  and  we  visited  together  the 
Arab  quarter  of  Gran.  The  flat-roofed  houses  appeared 
very  clean  and  white.  The  street  was  filled  with  loiter- 
ers, and  the  thresholds  were  occupied  by  picturesque 
groups.  Some  of  the  men  were  very  fine.  We  saw  many 
straight,  manly  fellows  who  must  have  been  six  feet  four 
in  height.  They  passed  us  with  perfect  indifference,  evinc- 
ing no  anger,  suspicion,  or  curiosity,  hardly  caring  in 
fact  to  glance  at  us  as  we  passed.  In  one  instance  only 
during  my  stay  at  Gran  was  I  spoken  to  by  an  Arab. 
He  was  a  tall,  good-humored  fellow,  who  came  smiling 
up  to  me,  and  muttered  something  about  "les  Anglais.'* 
The  mixed  population  of  Gran  is  picturesque  in  the  high- 
est degree:  the  Jews,  rich  and  poor,  varying  in  their 
costumes   as    their    wealth   varies;    the   Arabs   more   pict- 


f66  FRAGMENTS  OF  SCIENCE 

uresque  still,  and  of  all  shades  of  complexion — the  ne- 
groes, the  Spaniards,  the  French,  all  grouped  together, 
each  race  preserving  its  own  individuality,  formed  a  pict- 
ure intensely  interesting  to  me. 

On  Tuesday,  the  20th,  I  was  early  at  the  bastionet. 
The  night  had  been  very  squally.  The  sergeant  of  the 
sappers  had  taken  charge  of  our  key,  and  on  Tuesday 
morning  Elliot  went  for  it.  He  brought  back  the  intel- 
ligence that  the  tents  had  been  blown  down  and  the 
instruments  overturned.  Among  these  was  a  large  and 
valuable  equatorial  from  the  Eoyal  Observatory,  Green- 
wich. It  seemed  hardly  possible  that  this  instrument, 
with  its  wheels  and  verniers  and  delicate  adjustments, 
could  have  escaped  uninjured  from  such  a  fall.  This, 
however,  was  the  case;  and  during  the  day  all  the  over- 
turned instruments  were  restored  to  their  places,  and 
found  to  be  in  practical  working  order.  This  and  the 
following  day  were  devoted  to  incessant  schooling.  I  had 
come  out  as  a  general  star-gazer,  and  not  with  the  inten- 
tion of  devoting  myself  to  the  observation  of  any  par- 
ticular phenomenon.  I  wished  to  see  the  whole — the  first 
contact,  the  advance  of  the  moon,  the  successive  swallow- 
ing up  of  the  solar  spots,  the  breaking  of  the  last  line 
of  crescent  by  the  lunar  mountains  into  Bailey's  beads, 
the  advance  of  the  shadow  through  the  air,  the  appear- 
ance of  the  corona  and  prominences  at  the  moment  of 
totality,  the  radiant  streamers  of  the  corona,  the  internal 
structure  of  the  flames,  a  glance  through  a  polariscope, 
a  sweep  round  the  landscape  with  the  naked  eye,  the 
reappearance  of  the  solar  limb  through  Bailey's  beads, 
and,  finally,  the  retreat  of  the  lunar  shadow  through  the 
air. 


VOYAGE   TO   ALGERIA  167 

I  was  provided  with  a  telescope  of  admirable  defini- 
tion, mounted,  adjusted,  packed,  and  most  liberally  placed 
at  my  disposal  by  Mr.  "Warren  De  La  Kue.  The  telescope 
grasped  the  whole  of  the  sun,  and  a  considerable  portion 
of  the  space  surrounding  it.  But  it  would  not  take  in 
the  extreme  limits  of  the  corona.  For  this  I  had  lashed 
on  to  the  large  telescope  a  light  but  powerful  instrument, 
constructed  by  Eoss,  and  lent  to  me  by  Mr.  Huggins. 
I  was  also  furnished  with  an  excellent  binocular  by  Mr, 
Dallmeyer.  In  fact,  no  man  could  have  been  more  effi- 
ciently supported.  It  required  a  strict  parcelling  out  of 
the  interval  of  totality  to  embrace  in  it  the  entire  series 
of  observations.  These,  while  the  sun  remained  visible, 
were  to  be  made  with  an  unsilvered  diagonal  eye-piece, 
which  reflected  but  a  small  fraction  of  the  sun's  light,  this 
fraction  being  still  further  toned  down  by  a  dark  glass. 
At  the  moment  of  totality  the  dark  glass  was  to  be  re- 
moved, and  a  silver  reflector  pushed  in,  so  as  to  get  the 
maximum  of  light  from  the  corona  and  prominences.  The 
time  of  totality  was  distributed  as  follows: 


1.  Observe  approach  of  shadow  through  the  air:    totality 

2.  Telescope 30  seconds 

3.  Finder 30  seconds 

4.  Double  image  prism                              ,         ,         15  seconds 
6.  Naked  eye 10  seconds 

6.  Finder  or  binocular 20  seconds 

7.  Telescope .20  seconds 

8.  Observe  retreat  of  shadow 


Id  our  rehearsals  Elliot  stood  beside  me,  watch  in 
hand,  and  furnished  with  a  lantern.  He  called  out  at 
the  end  of  each  interval,  while  I  moved  from  telescope 
to  finder,  from  finder  to  polariscope,  from  polariscope  to 


168  FRAGMENTS   OF  SCIENCE 

naked  eye,  from  naked  eye  back  to  finder,  from  finder 
to  telescope,  abandoning  the  instrument  finally  to  observe 
the  retreating  shadow.  All  this  we  went  over  twenty 
times,  while  looking  at  the  actual  sun,  and  keeping  him 
in  the  middle  of  the  field.  It  was  my  object  to  render 
the  repetition  of  the  lesson  so  mechanical  as  to  leave  no 
room  for  flurry,  forgetfulness,  or  excitement.  Volition 
was  not  to  be  called  upon,  nor  judgment  exercised,  but 
a  well- beaten  path  of  routine  was  to  be  followed.  Had 
the  opportunity  occurred,  I  think  the  programme  would 
have  been  strictly  carried  out. 

But  the  opportunity  did  not  occur.  For  several  days 
the  weather  had  been  ill-natured.  We  had  wind  so  strong 
as  to  render  the  hawsers  at  the  stern  of  the  "Urgent"  as 
rigid  as  iron,  and  to  destroy  the  navigating  lieutenant's 
sleep.  We  had  clouds,  a  thunder-storm,  and  some  rain. 
Still  the  hope  was  held  out  that  the  atmosphere  would 
cleanse  itself,  and  if  it  did  we  were  promised  air  of  ex- 
traordinary limpidity.  Early  on  the  22 d  we  were  all  at 
our  posts.  Spaces  of  blue  in  the  early  morning  gave  us 
some  encouragement,  but  all  depended  on  the  relation 
of  these  spaces  to  the  surrounding  clouds.  Which  of 
them  were  to  grow  as  the  day  advanced?  The  wind 
was  high,  and  to  secure  the  steadiness  of  my  instru- 
ment I  was  forced  to  retreat  behind  a  projection  of  the 
bastionet,  place  stones  upon  its  stand,  and,  further,  to 
avail  myself  of  the  shelter  of  a  sail.  My  practiced  men 
fastened  the  sail  at  the  top,  and  loaded  it  with  bowlders 
at  the  bottom.     It  was  tried  severely,  but  it  stood  firm. 

The  clouds  and  blue  spaces  fought  for  a  time  with 
varying  success.  The  sun  was  hidden  and  revealed  at 
intervals,  hope  oscillating  in  synchronism  with  the  changes 


VOYAGE    TO   ALGERIA  169 

of  the  sky.  At  tlie  moment  of  first  contact  a  dense  clond 
intervened;  but  a  minute  or  two  afterward  the  cloud  had 
passed,  and  the  encroachment  of  the  black  body  of  the 
moon  was  evident  upon  the  solar  disk.  The  moon  marched 
onward,  and  I  saw  it  at  frequent  intervals ;  a  large  group 
of  spots  were  approached  and  swallowed  up.  Subse- 
quently I  caught  sight  of  the  lunar  limb  as  it  cut  through 
the  middle  of  a  large  spot.  The  spot  was  not  to  be  dis- 
tinguished from  the  moon,  but  rose  like  a  mountain  above 
it.  The  clouds,  when  thin,  could  be  seen  as  gray  scud 
drifting  across  the  black  surface  of  the  moon;  but  they 
thickened  more  and  more,  and  made  the  intervals  of  clear- 
ness scantier.  During  these  moments  I  watched  with  an 
interest  bordering  upon  fascination  the  march  of  the  silver 
sickle  of  the  sun  across  the  field  of  the  telescope.  It  was 
so  sharp  and  so  beautiful.  No  trace  of  the  lunar  limb 
could  be  observed  beyond  the  sun's  boundary.  Here,  in- 
deed, it  could  only  be  relieved  by  the  corona,  which  was 
utterly  cut  off  by  the  dark  glass.  The  blackness  of  the 
moon  beyond  the  sun  was,  in  fact,  confounded  with  the 
blackness  of  space. 

Beside  me  was  Elliot  with  the  watch  and  lantern,  while 
Lieutenant  Archer,  of  the  Koyal  Engineers,  had  the  kind- 
ness to  take  charge  of  my  note-book.  1  mentioned,  and 
he  wrote  rapidly  down,  such  things  as  seemed  worthy  of 
remembrance.  Thus  my  hands  and  mind  were  entirely 
free;  but  it  was  all  to  no  purpose.  A  patch  of  sunlight 
fell  and  rested  upon  the  landscape  some  miles  away.  It 
was  the  only  illuminated  spot  within  view.  But  to  the 
northwest  there  was  still  a  space  of  blue  which  might 
reach  us  in  time.  Within  seven  minutes  of  totality  an- 
other space  toward  the  zenith  became   very  dark.     The 

SOIENOB— V  —8 


170  FRAGMENTS   OF  SCIENCE 

atmosphere  was,  as  it  were,  on  the  brink  of  a  precipice, 
being  charged  with  humidity,  which  required  but  a  slight 
chill  to  bring  it  down  in  clouds.  This  was  furnished  by 
the  withdrawal  of  the  solar  beams:  the  clouds  did  come 
down,  covering  up  the  space  of  blue  on  which  our  hopes 
had  so  long  rested.  1  abandoned  the  telescope  and  walked 
to  and  fro  in  despair.  As  the  moment  of  totality  ap- 
proached, the  descent  toward  darkness  was  as  obvious  as 
a  falling  stone.  I  looked  toward  a  distant  ridge,  where 
the  darkness  would  first  appear.  At  the  moment  a  fan  of 
beams,  issuing  from  the  hidden  sun,  was  spread  out  over 
the  southern  heavens.  These  beams  are  bars  of  alternate 
light  and  shade,  produced  in  illuminated  haze  by  the 
shadows  of  floating  cloudlets  of  varying  density.  The 
beams  are  practically  parallel,  but  by  an  effect  of  per- 
spective they  appear  divergent,  having  the  sun,  in  fact, 
for  their  point  of  convergence.  The  darkness  took  posses- 
sion of  the  ridge  referred  to,  lowered  upon  M.  Janssen's 
observatory,  passed  over  the  southern  heavens,  blotting 
out  the  beams  as  if  a  sponge  had  been  drawn  across  them. 
It  then  took  successive  possession  of  three  spaces  of  blue 
sky  in  the  southeastern  atmosphere.  I  again  looked  to- 
ward the  ridge.  A  glimmer  as  of  day- dawn  was  behind 
it,  and  immediately  afterward  the  fan  of  beams,  which  had 
been  for  more  than  two  minutes  absent,  revived.  The 
eclipse  of  1870  had  ended,  and,  as  far  as  the  corona  and 
flames  were  concerned,  we  had  been  defeated. 

Even  in  the  heart  of  the  eclipse  the  darkness  was  by 
no  means  perfect.  Small  print  could  be  read.  In  fact, 
the  clouds  which  rendered  the  day  a  dark  one,  by  scatter- 
ing light  into  the  shadow,  rendered  the  darkness  less  in- 
tense than  it  would  have  been  had  the  atmosphere  been 


VOYAGE    TO    ALGERIA  171 

without  cloud.  In  the  more  open  spaces  I  sought  for 
stars,  but  could  find  none.  There  was  a  lull  in  the  wind 
before  and  after  totality,  but  during  the  totality  the  wind 
was  strong.  I  waited  for  some  time  on  the  bastion  et,  hop- 
ing to  get  a  glimpse  of  the  moon  on  the  opposite  border 
of  the  sun,  but  in  vain.  The  clouds  continued,  and  some 
rain  fell.  The  day  brightened  somewhat  afterward,  and, 
having  packed  all  up,  in  the  sober  twilight  Mr.  Crookes 
and  myself  climbed  the  heights  above  the  fort  of  Yera 
Cruz.  From  this  eminence  we  had  a  very  noble  view  over 
the  Mediterranean  and  the  flanking  African  hills.  The 
sunset  was  remarkable,  and  the  whole  outlook  exceed- 
mgly  fine. 

The  able  and  well-instructed  medical  officer  of  the 
''^ Urgent,"  Mr.  Goodman,  observed  the  following  tem- 
peratures during  the  progress  of  the  eclipse: 


Hour 

Deg. 

Hour 

Deg. 

11.45 

56 

12.43 

51 

11.55 

55 

1.05 

52 

12.10 

54 

1.27 

53 

12.37 

53 

1.44 

56 

12.39 

52 

2.10 

57 

The  minimum  temperature  occurred  some  minutes  after 
totality,  when  a  slight  rain  fell. 

The  wind  was  so  strong  on  the  23d  that  Captain  Hen- 
derson would  not  venture  out.  Guided  by  Mr.  Goodman, 
I  visited  a  cave  in  a  remarkable  stratum  of  shell- breccia, 
and,  thanks  to  my  guide,  secured  specimens.  Mr.  Busk 
informs  me  that  a  precisely  similar  breccia  is  found  at 
Gibraltar,  at  approximately  the  same  level.  During  the 
afternoon.  Admiral  Ommaney  and  myself  drove  to  the  fort: 
of  Marsa  el  Kibir      The  fortification  is  of  ancient  origin,, 


172  FRAGMENTS   OF  SCIENCE 

the  Moorisli  arches  being  still  there  in  decay,  but  the  fort 
is  now  very  strong.  About  four  or  five  hundred  fine- 
looking  dragoons  were  looking  after  their  horses,  waiting 
for  a  lull  to  enable  them  to  embark  for  France.  One  of 
their  officers  was  wandering  in  a  very  solitary  fashion  over 
the  fort.  We  had  some  conversation  with  him.  He  had 
been  at  Sedan,  had  been  taken  prisoner,  but  had  effected 
his  escape.  He  shook  his  head  when  we  spoke  of  the  ter- 
mination of  the  war,  and  predicted  its  long  continuance. 
There  was  bitterness  in  his  tone  as  he  spoke  of  the  charges 
of  treason  so  lightly  levelled  against  French  commanders. 
The  green  waves  raved  round  the  promontory  on  which 
the  fort  stands,  smiting  the  rocks,  breaking  into  foam,  and 
jumping,  after  impact,  to  a  height  of  a  hundred  feet  and 
more  into  the  air.  As  we  returned  our  vehicle  broke 
down  through  the  loss  of  a  wheel.  The  Admiral  went  on 
board,  while  I  remained  long  watching  the  agitated  sea. 
The  little  horses  of  Oran  well  merit  a  passing  word.  Their 
speed  and  endurance,  both  of  which  are  heavily  drawn 
upon  by  their  drivers,  are  extraordinary. 

The  wind  sinking,  we  lifted  anchor  on  the  24th.  For 
some  hours  we  went  pleasantly  along;  but  during  the 
afternoon  the  storm  revived,  and  it  blew  heavily  against 
us  all  the  night.  When  we  came  opposite  the  Bay  of 
Almeria,  on  the  25th,  the  captain,  turned  the  ship,  and 
steered  into  the  bay,  where,  under  the  shadow  of  the 
Sierra  Nevada,  we  passed  Christmas  night  in  peace.  Next 
morning  "a  rose  of  dawn"  rested  on  the  snows  of  the  ad- 
jacent mountains,  while  a  purple  haze  was  spread  over  the 
lower  hills.  1  had  no  notion  that  Spain  possessed  so  fine 
a  range  of  mountains  as  the  Sierra  Nevada.  The  height 
is  considerable,  but  the  form  also   is   such  as  to  get  the 


VOYAGE   TO    ALGERIA  ITS 

maxiimim  of  grandeur  out  of  tlie  height.  We  weighed 
anchor  at  eight  a.m.,  passing  for  a  time  through  shoal 
water,  the  bottom  having  been  evidently  stirred  up.  The 
adjacent  land  seemed  eroded  in  a  remarkable  manner.  It 
has  its  floods,  which  excavate  these  valleys  and  ravines, 
and  leave  those  singular  ridges  behind.  Toward  evening 
I  climbed  the  mainmast,  and,  standing  on  the  cross-trees, 
saw  the  sun  set  amid  a  blaze  of  fiery  clouds.  The  wind 
was  strong  and  bitterly  cold,  and  I  was  glad  to  slide  back 
to  the  deck  along  a  rope,  which  stretched  from  the  mast- 
head to  the  ship's  side.  That  night  we  cast  anchor  beside 
the  Mole  of  Gibraltar. 

On  the  morning  of  the  27th,  in  company  with  two 
friends,  I  drove  to  the  Spanish  lines,  with  the  view  of 
seeing  the  rock  from  that  side.  It  is  an  exceedingly  noble 
mass.  The  Peninsular  and  Oriental  mail-boat  had  been 
signalled  and  had  come.  Heavy  duties  called  me  home- 
ward, and  by  transferring  myself  from  the  "Urgent"  to 
the  mail- steamer  I  should  gain  three  days.  I  hired  a 
boat,  rowed  to  the  steamer,  learned  that  she  was  to  start 
at  one,  and  returned  with  all  speed  to  the  "Urgent." 
Making  known  to  Captain  Henderson  my  wish  to  get 
away,  he  expressed  doubts  as  to  the  possibility  of  reach- 
ing the  mail-steamer  in  time.  With  his  accustomed  kind- 
ness, he,  however,  placed  a  boat  at  my  disposal.  Four 
hardy  fellows  and  one  of  the  ship's  officers  jumped  intp 
it;  my  luggage,  hastily  thrown  together,  was  tumbled  in, 
and  we  were  immediately  on  our  way.  We  had  nearly 
four  miles  to  row  in  about  twenty  minutes ;  but  we  hoped 
the  mail-boat  might  not  be  punctual.  For  a  time  we 
watched  her  anxiously;  there  was  no  motion;  we  came 
nearer,   but  the  flags  were  no4i  yet  hauled  in.     The  men 


174  FRAGMENTS   OF  SCIENCE 

put  forth  all  their  strength,  animated  by  the  exhortations 
of  the  officer  at  the  helm.  The  roughness  of  the  sea  ren- 
dered their  efforts  to  some  extent  nugatory:  still  we  were 
rapidly  approaching  the  steamer.  At  length  she  moved, 
punctual  almost  to  the  minute,  at  first  slowly,  but  soon 
with  quickened  pace.  We  turned  to  the  left,  so  as  to  cut 
across  her  bows.  Five  minutes'  pull  would  have  brought 
us  up  to  her.  The  officer  waved  his  cap  and  I  my  hat. 
**If  they  could  only  see  us,  they  might  back  to  us  in  a 
moment."  But  they  did  not  see  us,  or  if  they  did,  they 
paid  us  no  attention.  I  returned  to  the  "Urgent,"  dis- 
comfited, but  grateful  to  the  fine  fellows  who  had  wrought 
so  hard  to  carry  out  my  wishes. 

Glad  of  the  quiet,  in  the  sober  afternoon  I  took  a  walk 
toward  Europa  Point.  The  sky  darkened  and  heavy 
squalls  passed  at  intervals.  Private  theatricals  were  at 
the  Convent,  and  the  kind  and  courteous  Governor  had 
sent  cards  to  the  eclipse  party.  I  failed  in  my  duty  in 
not  going.  St.  Michael's  Cave  is  said  to  rival,  if  it  does 
not  outrival,  the  Mammoth  Cave  of  Kentucky.  On  the 
28th  Mr.  Crookes,  Mr.  Carpenter,  and  myself,  guided  by 
a  military  policeman  who  understood  his  work,  explored 
the  cavern.  The  mouth  is  about  1,100  feet  above  the  sea. 
We  zigzagged  up  to  it,  and  first  were  led  into  an  aperture 
in  the  rock,  at  some  height  above  the  true  entrance  of 
the  cave.  In  this  upper  cavern  we  saw  some  tall  and 
beautiful  stalactite  pillars. 

The  water  drips  from  the  roof  charged  with  bicarbonate 
of  lime.  Exposed  to  the  air,  the  carbonic  acid  partially 
escapes,  and  the  simple  carbonate  of  lime,  which  is  hardly 
at  all  soluble  in  water,  deposits  itself  as  a  solid,  forming 
stalactites  and  stalagmites      Even  the  exposure  of  chalk 


VOYAGE   TO   ALGERIA  175 

or  limestone  water  to  the  open  air  partially  softens  it.  A 
specimen  of  the  Redboume  water  exposed  by  Professors 
Graham,  Miller,  and  Hofmann,  in  a  shallow  basin,  fell 
from  eighteen  degrees  to  nine  degrees  of  hardness.  The 
softening  process  of  Clark  is  virtually  a  hastening  of  the 
natural  process.  Here,  however,  instead  of  being  per- 
mitted to  evaporate,  half  the  carbonic  acid  is  appropriated 
by  lime,  the  half  thus  taken  up,  as  well  as  the  remaining 
half,  being  precipitated.  The  solid  precipitate  is  per- 
mitted to  sink,  and  the  clear  supernatant  liquid  is  limpid 
Boft  water. 

We  returned  to  the  real  mouth  of  St.  Michaers  Cave, 
which  is  entered  by  a  wicket.  The  floor  was  somewhat 
muddy,  and  the  roof  and  walls  were  wet.  We  soon  found 
ourselves  in  the  midst  of  a  natural  temple,  where  tall  col- 
umns sprang  complete  from  floor  to  roof,  while  incipient 
columns  were  growing  to  meet  each  other,  upward  and 
downward.  The  water  which  trickles  from  the  stalactite, 
after  having  in  part  yielded  up  its  carbonate  of  lime,  falls 
upon  the  floor  vertically  underneath,  and  there  builds  the 
stalagmite.  Consequently,  the  pillars  grow  from  above 
and  below  simultaneously,  along  the  same  vertical.  It 
is  easy  to  distinguish  the  stalagmitic  from  the  stalactitic 
portion  of  the  pillars.  The  former  is  always  divided  into 
short  segments  by  protuberant  rings,  as  if  deposited  peri- 
odically, while  the  latter  presents  a  uniform  surface.  In 
some  cases  the  points  of  inverted  cones  of  stalactite  rested 
on  the  centres  of  pillars  of  stalagmite.  The  process  of 
solidification  and  the  consequent  architecture  were  alike 
beautiful. 

We  followed  our  guide  through  various  branches  and 
arms  of  the  cave,  climbed  and  descended  steps,  halted  at 


176  FRAGMENTS    OF  SCIENCE 

the  edges  of  dark  shafts  and  apertures,  and  squeezed  our- 
selves through  narrow  passages.  From  time  to  time  we 
halted,  while  Mr.  Crookes  illuminated,  with  ignited  mag- 
nesium wire,  the  roof,  columns,  dependent  spears,  and 
graceful  drapery  of  the  stalactites.  Once,  coming  to  a 
magnificent  cluster  of  icicle-like  spears,  we  helped  our- 
selves to  specimens.  There  was  some  difficulty  in  detach- 
ing the  more  delicate  ones,  their  fragility  was  so  great. 
A  consciousness  of  vandalism,  which  smote  me  at  the 
time,  haunts  me  still;  for,  though  our  requisitions  were 
moderate,  this  beauty  ought  not  to  be  at  all  invaded. 
Pendent  from  the  roof,  in  their  natural  habitat,  nothing 
can  exceed  their  delicate  beauty;  they  live^  as  it  were,  sur- 
rounded by  organic  connections.  In  London  they  are 
curious,  but  not  beautiful.  Of  gathered  shells  Emerson 
writes: 

I  wiped  away  the  weeds  and  foam. 
And  brought  my  sea-born  treasures  home: 
But  the  poor,  unsightly,  noisome  things 
Had  left  their  beauty  on  the  shore, 
With  the  sun,  and  the  sand,  and  the  wild  uproar. 

The  promontory  of  Gibraltar  is  so  burrowed  with  cav- 
erns that  it  has  been  called  the  Hill  of  Caves.  They  are 
apparently  related  to  the  geologic  disturbances  which  the 
rock  has  undergone.  The  earliest  of  these  is  the  tilting 
of  the  once  horizontal  strata.  Suppose  a  force  of  torsion 
to  act  upon  the  promontory  at  its  southern  extremity  near 
Europa  Point,  and  suppose  the  rock  to  be  of  a  partially 
yielding  character;  such  a  force  would  twist  the  strata 
into  screw- surfaces,  the  greatest  amount  of  twisting  being 
endured  near  the  point  of  application  of  the  force.  Such 
a  twisting  the  rock  appears  to  have  suffered*,  but  instead 


VOYAGE    TO    ALGERIA  177 

of  the  twist  fading  gradually  and  uniformly  off,  in  passing 
from  south  to  north,  the  want  of  uniformity  in  the  material 
has  produced  lines  of  dislocation  where  there  are  abrupt 
changes  in  the  amount  of  twist.  Thus,  at  the  northern 
end  of  the  rock  the  dip  to  the  west  is  nineteen  degrees; 
in  the  Middle  Hill,  it  is  thirty-eight  degrees;  in  the  centre 
of  the  South  Hill,  or  Sugar  Loaf,  it  is  fifty-seven  degrees. 
At  the  southern  extremity  of  the  Sugar  Loaf  the  strata 
are  vertical,  while  further  to  the  south  they  actually  turn 
over  and  dip  to  the  east. 

The  rock  is  thus  divided  into  three  sections,  separated 
from  each  other  by  places  of  dislocation,  where  the  strata 
are  much  wrenched  and  broken.  These  are  called  the 
Northern  and  Southern  Quebrada,  from  the  Spanish 
"Tierra  Quebrada,"  or  broken  ground.  It  is  at  these 
places  that  the  inland  caves  of  Gibraltar  are  almost  exclu- 
sively found.  Based  on  the  observations  of  Dr.  Falconer 
and  himself,  an  excellent  and  most  interesting  account  of 
these  caves,  and  of  the  human  remains  and  works  of  art 
which  they  contain,  was  communicated  by  Mr.  Busk  to 
the  meeting  of  the  Congress  of  Prehistoric  Archosology 
at  Norwich,  arid  afterward  printed  in  the  "Transactions" 
of  the  Congress.*  Long  subsequent  to  the  operation  of 
the  twisting  force  just  referred  to,  the  promontory  under- 
went various  changes  of  level.  There  are  sea-terraces  and 
layers  of  shell-breccia  along  its  flanks,  and  numerous  caves 
which,  unlike  the  inland  ones,  are  the  product  of  marine 
erosion.     The    Ape's    Hill,    on   the   African    side    of    the 


*  In  this  essay  Mr.  Busk  refers  to  the  previous  labors  of  Mr.  Smith,  of 
Jordan  Hill,  to  whom  we  owe  most  of  our  knowledge  of  the  geology  of  the 
rock. 


178  FRAGMENTS    OF   SCIENCE 

strait,    Mr.    Busk    informs    me,    has    undergone    similar 
disturbances.  * 

In  the  harbor  of  Gibraltar,  on  the  morning  of  our  de- 
parture, I  resumed  a  series  of  observations  on  the  color 
of  the  sea.  On  the  way  out  a  number  of  specimens  had 
been  collected,  with  a  view  to  subsequent  examination. 
But  the  bottles  were  claret  bottles,  of  doubtful  purity. 
At  Gibraltar,  therefore,  I  purchased  fifteen  white  glass 
bottles,  with  ground  glass  stoppers,  and  at  Cadiz,  thanks 
to  the  friendly  guidance  of  Mr.  Cameron,  I  secured  a 
dozen  more.  These  seven- and- twenty  bottles  were  filled 
with  water,  taken  at  different  places  between  Oran  and 
Spithead. 

And  here  let  me  express  my  warmest  acknowledg- 
ments to  Captain  Henderson,  the  commander  of  H.M.S. 
* 'Urgent,"  who  aided  me  in  my  observations  in  every 
possible  way.  Indeed,  my  thanks  are  due  to  all  the  offi- 
cers for  their  unfailing  courtesy  and  help.  The  captain 
placed  at  my  disposal  his  own  coxswain,  an  intelligent 
fellow  named  Thorogood,  who  skilfully  attached  a  cord 
to  each  bottle,  weighted  it  with  lead,  cast  it  into  the  sea, 
and,  after  three  successive  rinsings,  filled  it  under  my 
own  eyes.  The  contact  of  jugs,  buckets,  or  other  vessels 
was  thus  avoided;  and  even  the  necessity  of  pouring  out 
the  water,  afterward,  through  the  dirty  London  air. 

The  mode  of  examination  applied  to  these  bottles  has 
been  already  described."     The  liquid  is  illuminated  by  a 

^  No  one  can  rise  from  the  perusal  of  Mr.  Busk's  paper  without  a  feeling 
of  admiration  for  the  principal  discoverer  and  indefatigable  explorer  of  the 
Gibraltar  caves,  the  late  Captain  Frederick  Brome. 

2  "Floating  Matter  of  the  Air,"  Art.  "Dust  and  Disease." 


VOYAGE   TO   ALGERIA 


179 


powerfully  condensed  beam,  its  condition  being  revealed 
through  the  light  scattered  by  its  suspended  particles. 
•*Care  is  taken  to  defend  the  eye  from  the  access  of  all 
other  light,  and,  thus  defended,  it  becomes  an  organ  of 
inconceivable  delicacy.*'  Were  water  of  uniform  density 
perfectly  free  from  suspended  matter,  it  would,  in  my 
opinion,  scatter  no  light  at  all.  The  track  of  a  luminous 
beam  could  not  be  seen  in  such  water.  But  **an  amount 
of  impurity  so  infinitesimal  as  to  be  scarcely  expressible 
in  numbers,  and  the  individual  particles  of  which  are  so 
small  as  wholly  to  elude  the  microscope,  may,  when  ex- 
amined by  the  method  alluded  to,  produce  not  only  sen- 
sible, but  striking,  effects  upon  the  eye." 

The  results  of  the  examination  of  nineteen  bottles  filled 
at  various  places  between  Gibraltar  and  Spithead  are  here 
tabulated: 


No. 

LocaUty 

Color  of  Sea 

Appearance  in  Luminous  Beam 

1 

Gibraltar  Harbor 

Green 

Thick  with  fine  particles 

2 

Two  miles  from  Gibraltar 

Clearer  green 

Thick  with  very  fine  particles 

3 

Off  Cabreta  Point 

Bright  green 

Still  thick,  but  less  so 

4 

Off  Cabreta  Point 

Black-indigo 

Much  less  thick,  very  pure 

5 

Off  Tarifa 

Undecided 

Thicker  than  No,  4 

6 

Beyond  Tarifa 

Cobalt-blue 

Much  purer  than  No.  5 
Very  thick 

7 

Twelve  miles  from  Cadiz 

Yellow-green 

8 

Cadiz  Harbor 

Yellow-green 

Exceedingly  thick 
Thick,  but  less  so 

9 

Fourteen  miles  from  Cadiz 

Yellow-green 

10 

Fourteen  miles  from  Cadiz 

Bright  green 

Much  less  thick 

11 

Between  Capes  St.  Mary  and 

Vincent 

Deep  indigo 

Very  little  matter,  very  pure 

12 

Off  the  Burlings 

Strong  green 

Thick,  with  fine  matter 

13 

Beyond  the  Burlings 
Off  Cape  Finisterre 

Indigo 

Very  little  matter,  pure 

14 

Undecided 

Less  pure 

Very  little  matter,  very  pm^ 

15 

Bay  of  Biscay 

Black-indigo 

16 

Bay  of  Biscay 
Off  Ushant 

Indigo 

Very  fine  matter.    Iridescent 

17 

Dark  green 

A  good  deal  of  matter 

18 

Off  St.  Catherine's 

Yellow-green 

Exceedingly  thick 

19 

Spithead 

Green 

Exceedingly  thick 

Here  we  have  three  specimens  of  water,  described  as 
green,  a  clearer  green,  and  bright  green,  taken  in  Gibral- 
tar Harbor,  at  a  point  two  miles  from  the  harbor,  and  off 


180  FRAGMENTS   OF  SCIENCE 

Cabreta  Point.  The  home  examination  showed  the  first 
to  be  thick  with  suspended  matter,  the  second  less  thick, 
and  the  third  still  less  thick.  Thus  the  green  brightened 
as  the  suspended  matter  diminished  in  amount. 

Previous  to  the  fourth  observation  our  excellent  nav- 
igating lieutenant,  Mr.  Brown,  steered  along  the  coast, 
thus  avoiding  the  adverse  current  which  sets  in,  through 
the  Strait,  from  the  Atlantic  to  the  Mediterranean.  He 
was  at  length  forced  to  cross  the  boundary  of  the  Atlantic 
current,  which  was  defined  with  extraordinary  sharpness. 
On  the  one  side  of  it  the  water  was  a  vivid  green,  on  the 
other  a  deep  blue.  Standing  at  the  bow  of  the  ship,  a 
bottle  could  be  filled  with  blue  water,  while  at  the  same 
moment  a  bottle  cast  from  the  stern  could  be  filled  with 
green  water.  Two  bottles  were  secured,  one  on  each  side 
of  this  remarkable  boundary.  In  the  distance  the  At- 
lantic had  the  hue  called  ultramarine;  but  looked  fairly 
down  upon,  it  was  of  almost  inky  blackness — black  quali- 
fied by  a  trace  of  indigo. 

What  change  does  the  home  examination  here  reveal? 
In  passing  to  indigo,  the  water  becomes  suddenly  aug- 
mented in  purity,  the  suspended  matter  becoming  sud- 
denly less.  Off  Tarifa,  the  deep  indigo  disappears,  and 
the  sea  is  undecided  in  color.  Accompanying  this  change, 
we  have  a  rise  in  the  quantity  of  suspended  matter.  Be- 
yond Tarifa,  we  change  to  cobalt-blue,  the  suspended 
matter  falling  at  the  same  time  in  quantity.  This  water 
is  distinctly  purer  than  the  green.  We  approach  Cadiz, 
and  at  twelve  miles  from  the  city  get  into  yellow- green 
water;  this  the  London  examination  shows  to  be  thick 
with  suspended  matter.  The  same  is  true  of  Cadiz  Har- 
bor, and  also  of  a  point  fourteen  miles  from  Cadiz  in  the 


VOYAGE    TO   ALGERIA  181 

homeward  direction.  Here  there  is  a  sudden  change  from 
yellow-green  to  a  bright  emerald-green,  and  accompany- 
ing the  change  a  sudden  fall  in  the  quantity  of  suspended 
matter.  Between  Cape  St.  Mary  and  Cape  St.  Vincent 
the  water  changes  to  the  deepest  indigo,  a  further  dimi- 
nution of  the  suspended  matter  being  the  concomitant 
phenomenon. 

"We  now  reach  the  remarkable  group  of  rocks  called 
the  Burlings,  and  find  the  water  between  the  shore  and 
the  rocks  a  strong  green;  the  home  examination  shows 
it  to  be  thick  with  fine  matter.  Fifteen  or  twenty  miles 
beyond  the  Burlings  we  come  again  into  indigo  water, 
from  which  the  suspended  matter  has  in  great  part  dis- 
appeared. Ofl*  Cape  Finisterre,  about  the  place  where  the 
"Captain"  went  down,  the  water  becomes  green,  and  the 
home  examination  pronounces  it  to  be  thicker.  Then  we 
enter  the  Bay  of  Biscay,  where  the  indigo  resumes  its 
power,  and  where  the  home  examination  shows  the  greatly 
augmented  purity  of  the  water.  A  second  specimen  of 
water,  taken  from  the  Bay  of  Biscay,  held  in  suspension 
fine  particles  of  a  peculiar  kind;  the  size  of  them  was 
such  as  to  render  the  water  richly  iridescent.  It  showed 
itself  green,  blue,  or  salmon-colored,  according  to  the  di- 
rection of  the  line  of  vision.  Finally,  we  come  to  our  last 
two  bottles,  the  one  taken  opposite  St.  Catherine's  light- 
house, in  the  Isle  of  Wight,  the  other  at  Spithead.  The 
sea  at  both  these  places  was  green,  and  both  specimens, 
as  might  be  expected,  were  pronounced  by  the  home 
examination  to  be  thick  with  suspended  matter. 

Two  distinct  series  of  observations  are  here  referred 
to — the  one  consisting  of  direct  observations  of  the  color 
of  the  sea,  conducted  during  the  voyage  from  Gibraltar  to 


182  FRAGMENTS   OF  SCIENCE 

Portsmouth:  the  other  carrisd  out  in  the  laboratory  of  the 
Eojal  Institution.  And  here  it  is  to  be  noted  that  in  the 
home  examination  I  never  knew  what  water  was  placed 
in  mj  hands.  The  labels,  with  the  names  of  the  localities 
written  upon  them,  had  been  tied  up,  all  information  re- 
garding the  source  of  the  water  being  thus  held  back. 
The  bottles  were  simply  numbered,  and  not  till  all  of 
them  had"  been  examined,  and  described,  were  the  labels 
opened,  and  the  locality  and  sea-color  corresponding  to 
the  various  specimens  ascertained.  The  home  observa- 
tions, therefore,  must  have  been  perfectly  unbiased,  and 
they  clearly  establish  the  association  of  the  green  color  with 
fine  suspended  matter,  and  of  the  ultramarine  color,  and 
more  especially  of  the  black-indigo  hue  of  the  Atlantic, 
with  the  comparative  absence  of  such  matter. 

So  much  for  mere  observation;  but  what  is  the  cause 
of  the  dark  hue  of  the  deep  ocean?*  A  preliminary  re- 
mark or  two  will  clear  our  way  toward  an  explanation. 
Color  resides  in  white  light,  appearing  when  any  constit- 
uent of  the  white  light  is  withdrawn.  The  hue  of  a  pur- 
ple liquid,  for  example,  is  immediately  accounted  for  by 
its  action  on  a  spectrum.  It  cuts  out  the  yellow  and 
green,  and  allows  the  red  and  blue  to  pass  through.  The 
blending  of  these  two  colors  produces  the  purple.  But 
while  such  a  liquid  attacks  with  special  energy  the  yellow 
and  green,  it  enfeebles  the  whole  spectrum.       By  increas- 


*  A  note,  written  to  me  on  October  22,  by  my  friend  Canon  Kingsley,  con- 
tains the  following  reference  to  this  point:  "I  have  never  seen  the  Lake  ol 
Geneva,  but  I  thought  of  the  brilliant  dazzling  dark  blue  of  the  mid- Atlantic 
under  the  sunlight,  and  its  black-blue  under  cloud,  both  so  solid  that  one  might 
leap  off  the  sponson  on  to  it  without  fear;  this  was  to  me  the  most  wonderful 
thing  whicli  I  saw  on  my  voyages  to  and  from  the  West  Indies.*' 


VOYAGE    TO    ALGERIA  183 

ing  the  thickness  of  the  stratum  we  may  absorb  the  whole 
of  the  light.  The  color  of  a  blue  liquid  is  similarly  ac- 
counted for.  It  first  extinguishes  the  red;  then,  as  the 
thickness  augments,  it  attacks  the  orange,  yellow,  and 
green  in  succession;  the  blue  alone  finally  remaining. 
But  even  it  might  be  extinguished  by  a  sufficient  depth 
of  the  liquid. 

And  now  we  are  prepared  for  a  brief,  but  tolerably 
complete,  statement  of  that  action  of  sea-water  upon  light 
to  which  it  owes  its  darkness.  The  spectrum  embraces 
three  classes  of  rays — the  thermal,  the  visual,  and  the 
chemical.  These  divisions  overlap  each  other;  the  ther- 
mal rays  are  in  part  visual,  the  visual  rays  in  part  chem- 
ical, and  vice  versa.  The  vast  body  of  thermal  rays  lie 
beyond  the  red,  being  invisible.  These  rays  are  attacked 
with  exceeding  energy  by  water.  They  are  absorbed  close 
to  the  surface  of  the  sea,  and  are  the  great  agents  in  evap- 
oration. At  the  same  time  the  whole  spectrum  suffers  en- 
feeblement;  water  attacks  all  its  rays,  but  with  different 
degrees  of  energy.  Of  the  visual  rays,  the  red  are  first 
extinguished.  As  the  solar  beam  plunges  deeper  into  the 
sea,  orange  follows  red,  yellow  follows  orange,  green  fol- 
lows yellow,  and  the  various  shades  of  blue,  where  the 
water  is  deep  enough,  follow  green.  Absolute  extinction 
of  the  solar  beam  would  be  the  consequence  if  the  water 
were  deep  and  uniform.  If  it  contained  no  suspended 
matter,  such  water  would  be  as  black  as  ink.  A  reflected 
glimmer  of  ordinary  light  would  reach  us  from  its  surface, 
as  it  would  from  the  surface  of  actual  ink;  but  no  light, 
hence  no  color,  would  reach  us  from  the  body  of  the 
water. 

In  very  clear  and  deep  sea- water  this  condition  is  ap- 


184  FRAGMENTS   OF  SCIENCE 

proximately  fulfilled,  and  hence  the  extraordinary  dark- 
ness of  such  water.  The  indigo,  already  referred  to,  is, 
I  believe,  to  be  ascribed  in  part  to  the  suspended  matter, 
which  is  never  absent,  even  in  the  purest  natural  water; 
and  in  part  to  the  slight  reflection  of  the  light  from  the 
limiting  surfaces  of  strata  of  different  densities.  A  modi- 
cum of  light  is  thus  thrown  back  to  the  eye,  before  the 
depth  necessary  to  absolute  extinction  has  been  attained. 
An  effect  precisely  similar  occurs  under  the  moraines  of 
glaciers.  The  ice  here  is  exceptionally  compact,  and, 
owing  to  the  absence  of  the  internal  scattering  common 
in  bubbled  ice,  the  light  plunges  into  the  mass,  where 
it  is  extinguished,  the  perfectly  clear  ice  presenting  an 
appearance  of  pitchy  blackness.* 

The  green  color  of  the  sea  has  now  to  be  accounted 
for;  and  here,  again,  let  us  fall  back  upon  the  sure  basis 
of  experiment.  A  strong  white  dinner-plate  had  a  lead 
weight  securely  fastened  to  it.  Fifty  or  sixty  yards  of 
strong  hempen  line  were  attached  to  the  plate.  My  assist,- 
ant,  Thorogood,  occupied  a  boat,  fastened  as  usual  to  the 
davits  of  the  "Urgent,"  while  I  occupied  a  second  boat 
nearer  the  stem  of  the  ship.  He  cast  the  plate  as  a  mari- 
ner heaves  the  lead,  and  by  the  time  it  reached  me  it  had 
sunk  a  considerable  depth  in  the  water.  In  all  cases  the 
hue  of  this  plate  was  green.  Even  when  the  sea  was  of 
the  darkest  indigo,  the  green  was  vivid  and  pronounced. 
I  could  notice  the  gradual  deepening  of  the  color  as  the 
plate  sank,  but  at  its  greatest  depth,  even  in  indigo  water, 
tiie  color  was  still  a  blue  green.' 

'  I  learn  from  a  correspondent  that  certain  Welsh  tarns,  which  are  reputed 
bottomless,  have  this  inky  hue. 

•  In  no  case,  of  coursQ  is  the  green  pure,  but  a  mixture  of  green  and  blue. 


VOYAGE    TO    ALGERIA  185 

Other  observations  confirmed  this  one.  The  *' Urgent" 
is  a  screw  steamer,  and  right  over  the  blades  of  the  screw 
was  an  orifice  called  the  screw-well,  through  which  one 
could  look  from  the  poop  down  upon  the  screw.  The  sur- 
face-glimmer, which  so  pesters  the  eye,  was  here  in  a  great 
measure  removed.  Midway  down,  a  plank  crossed  the 
screw- well  from  side  to  side;  on  this  I  placed  myself  and 
observed  the  action  of  the  screw  underneath.  The  eye 
was  rendered  sensitive  by  the  moderation  of  the  light; 
and,  to  remove  still  further  all  disturbing  causes,  Lieu- 
tenant Walton  had  a  sail  and  tarpaulin  thrown  over  the 
mouth  of  the  well.  Underneath  this  I  perched  myself  on 
the  plank  and  watched  the  screw.  In  an  indigo  sea  the 
play  of  color  was  indescribably  beautiful,  and  the  contrast 
between  the  water,  which  had  the  screw-blades,  and  that 
which  had  the  bottom  of  the  ocean,  as  a  background,  was 
extraordinary.  The  one  was  of  the  most  brilliant  green, 
the  other  of  the  deepest  ultramarine.  The  surface  of  the 
water  above  the  screw-blade  was  always  ruffled.  Liquid 
lenses  were  thus  formed,  by  which  the  colored  light  was 
withdrawn  from  some  places  and  concentrated  upon  others, 
the  water  flashing  with  metallic  lustre.  The  screw-blades 
in  this  case  played  the  part  of  the  dinner-plate  in  the 
former  case,  and  there  were  other  instances  of  a  similar 
kind.  The  white  bellies  of  porpoises  showed  the  green 
hue,  varying  in  intensity  as  the  creatures  swung  to  and 
fro  between  the  surface  and  the  deeper  water.  Foam,  at 
a  certain  depth  below  the  surface,  was  also  green.  In  a 
rough  sea  the  light  which  penetrated  the  summit  of  a  wave 
sometimes  reached  the  eye,  a  beautiful  green  cap  being 
thus  placed  upon  the  wave,  even  in  indigo  water. 

But  how  is  this    color  to  be  connected  with  the  sus- 


186  FRAGMENTS   OF  SCIENCE 

pended  particles?  Thus:  Take  tlie  dinner-plate  which 
showed  so  brilliant  a  green  when  thrown  into  indigo 
water.  Suppose  it  to  diminish  in  size,  until  it  reaches 
an  almost  microscopic  magnitude.  It  would  still  behave 
substantially  as  the  larger  plate,  sending  to  the  eye  its 
modicum  of  green  light.  If  the  plate,  instead  of  being  a 
large  coherent  mass,  were  ground  to  a  powder  sufficiently 
fine,  and  in  this  condition  diffused  through  the  clear  sea- 
water,  it  would  also  send  green  light  to  the  eye.  In  fact, 
the  suspended  particles  which  the  home  examination  re- 
veals act  in  all  essential  particulars  like  the  plate,  or  like 
the  screw-blades,  or  like  the  foam,  or  like  the  bellies  of 
the  porpoises.  Thus  I  think  the  greenness  of  the  sea  is 
physically  connected  with  the  matter  which  it  holds  in 
suspension. 

We  reached  Portsmouth  on  January  5,  1871.  Then 
ended  a  voyage  which,  though  its  main  object  was  not 
realized,  has  left  behind  it  pleasant  memories,  both  of  the 
aspects  of  nature  and  the  kindliness  of  men. 


VII 

NIAGARA  * 

IT  is  one  of  the  disadvantages  of  reading  books  about  nat- 
ural scenery  that  they  fill  the  mind  with  pictures,  often 
exaggerated,  often  distorted,  often  blurred,  and,  even 
when  well  drawn,  injurious  to  the  freshness  of  first  impres- 
sions. Such  has  been  the  fate  of  most  of  us  with  regard 
to  the  Falls  of  Niagara.  There  was  little  accuracy  m  the 
estimates  of  the  first  observers  of  the  cataract.  Startled 
by  an  exhibition  of  power  so  novel  and  so  grand,  emotion 
leaped  beyond  the  control  of  the  judgment,  and  gave  cur- 
rency to  notions  which  have  often  led  to  disappointment. 
A  record  of  a  voyage,  in  1535,  by  a  French  mariner 
named  Jacques  Cartier,  contains,  it  is  said,  the  first  printed 
allusion  to  Niagara.  In  1603  the  first  map  of  the  district 
was  constructed  by  a  Frenchman  named  Champlain.  In 
1648  the  Jesuit  Rageneau,  in  a  letter  to  his  superior  at 
Paris,  mentions  Niagara  as  "a  cataract  of  frightful 
height.*'*  In  the  winter  of  1678  and  1679  the  cataract 
was  visited  by  Father  Hennepin,  and  described  in  a  book 
dedicated  **to  the  King  of  Great  Britain."  He  gives  a 
drawing  of  the  waterfall,  which  shows  that  serious  changes 
have  taken  place  since  his  time.     He  describes  it  as  **a 

»  A  Discourse  delivered  at  the  Royal  Institution  of  Great  Britain,  April  4, 
18'73. 

'  From  an  interesting  little  book  presented  to  me  at  Brooklyn  by  its  author, 
Mr.  Holly,  some  of  these  data  are  derived:  Hennepin,  Kalm,  Bakewell,  Lyell, 
Hall,  and  others  I  have  myself  consulted. 

(187) 


188  FRAGMENTS   OF  SCIENCE 

great  and  prodigious  cadence  of  water,  to  wMch  the  iini- 
verse  does  not  offer  a  parallel."  The  height  of  the  fall, 
according  to  Hennepin,  was  more  than  600  feet.  "The 
waters,"  he  says,  "which  fall  from  this  great  precipice  do 
foam  and  boil  in  the  most  astonishing  manner,  making  a 
noise  more  terrible  than  that  of  thunder.  "When  the  wind 
blows  to  the  south  its  frightful  roaring  may  be  heard  for 
more  than  fifteen  leagues."  The  Baron  la  Hontan,  who 
visited  JSTiagara  in  1687,  makes  the  height  800  feet.  In 
1721  Charlevois,  in  a  letter  to  Madame  de  Maintenon,  after 
referring  to  the  exaggerations  of  his  predecessors,  thus 
states  the  result  of  his  own  observations:  "For  my  part, 
after  examining  it  on  all  sides,  I  am  inclined  to  think  that 
we  cannot  allow  it  less  than  140  or  150  feet" — a  remark- 
ably close  estimate.  At  that  time,  viz.,  a  hundred  and 
fifty  years  ago,  it  had  the  shape  of  a  horseshoe,  and  rea- 
sons will  subsequently  be  given  for  holding  that  this  has 
been  always  the  form  of  the  cataract,  from  its  origin  to 
its  present  site. 

As  regards  the  noise  of  the  fall,  Charlevois  declares 
the  accounts  of  his  predecessors,  which,  I  may  say,  are  re- 
peated to  the  present  hour,  to  be  altogether  extravagant. 
He  is  perfectly  right.  The  thunders  of  Niagara  are  for- 
midable enough  to  those  who  really  seek  them  at  the  base 
of  the  Horseshoe  Fall ;  but  on  the  banks  of  the  river,  and 
particularly  above  the  fall,  its  silence,  rather  than  its  noise, 
is  surprising.  This  arises,  in  part,  from  the  lack  of  reso- 
nance; the  surrounding  country  being  flat,  and  therefore 
furnishing  no  echoing  surfaces  to  reinforce  the  shock  of 
the  water.  The  resonance  from  the  surrounding  rocks 
causes  the  Swiss  Eeuss  at  the  Devil's  Bridge,  when  full, 
to  thunder  more  loudly  than  the  Niagara. 


NIAGARA  189 

On  Friday,  November  1,  1872,  just  before  reaching;  the 
village  of  Niagara  Falls,  I  caught,  from  the  railway  train, 
my  first  glimpse  of  the  smoke  of  the  cataract.  Immedi- 
ately after  my  arrival  I  went  with  a  friend  to  the  northern 
end  of  the  American  Fall.  It  may  be  that  my  mood  at 
the  time  toned  down  the  impression  produced  by  the  first 
aspect  of  this  grand  cascade;  but  I  felt  nothing  like  disap- 
pointment, knowing,  from  old  experience,  that  time  and 
close  acquaintanceship,  the  gradual  interweaving  of  mind 
and  nature,  must  powerfully  influence  my  final  estimate  ot 
the  scene.  After  dinner  we  crossed  to  Goat  Island,  and, 
turning  to  the  right,  reached  the  southern  end  of  the 
American  Fall.  The  river  is  here  studded  with  small 
islands.  Crossing  a  wooden  bridge  to  Luna  Island,  and 
clasping  a  tree  which  grows  near  its  edge,  I  looked  long 
at  the  cataract,  which  here  shoots  down  the  precipice  like 
an  avalanche  of  foam.  It  grew  in  power  and  beauty. 
The  channel  spanned  by  the  wooden  bridge  was  deep, 
and  the  river  there  doubled  over  the  edge  of  the  preci- 
pice, like  the  swell  of  a  muscle  unbroken.  The  ledge 
here  overhangs,  the  water  being  poured  out  far  beyond 
the  base  of  the  precipice.  A  space,  called  the  Cave  of 
the  Winds,  is  thus  enclosed  between  the  wall  of  rock  and 
the  falling  water. 

Goat  Island  ends  in  a  sheer  dry  precipice,  which  con- 
nects the  American  and  Horseshoe  Falls.  Midway  be- 
tween both  is  a  wooden  hut,  the  residence  of  the  guide 
to  the  Cave  of  the  Winds,  and  from  the  hut  a  winding 
staircase,  called  Biddle's  Stair,  descends  to  the  base  of 
the  precipice.  On  the  evening  of  my  arrival  I  went  down 
this  stair,  and  wandered  along  the  bottom  of  the  cliff. 
One  well-known  factor  in  the  formation  and  retreat  of  the 


190  FRAGMENTS    OF  SCIENCE 

cataract  was  immediately  observed.  A  thick  layer  of 
limestone  formed  tlie  upper  portion  of  the  cliff.  This 
rested  upon  a  bed  of  soft  shale,  which  extended  round 
the  base  of  the  cataract.  The  violent  recoil  of  the  water 
against  this  yielding  substance  crumbles  it  away,  under- 
mining the  ledge  above,  which,  unsupported,  eventually 
breaks  off,  and  produces  the  observed  recession. 

At  the  southern  extremity  of  the  Horseshoe  is  a  prom- 
ontory, formed  by  the  doubling  back  of  the  gorge  exca- 
vated by  the  cataract,  and  into  which  it  plunges.  On  the 
promontory  stands  a  stone  building,  called  the  Terrapin 
Tower,  the  door  of  which  had  been  nailed  up  because  of 
the  decay  of  the  staircase  within  it.  Through  the  kind- 
ness of  Mr.  Town  send,  the  superintendent  of  Goat  Island, 
the  door  was  opened  for  me.  From  this  tower,  at  all  hours 
of  the  day,  and  at  some  hours  of  the  night,  I  watched  and 
listened  to  the  Horseshoe  Fall.  The  river  here  is  evi- 
dently much  deeper  than  the  American  branch:  and  in- 
stead of  bursting  into  foam  where  it  quits  the  ledge,  it 
bends  solidly  over,  and  falls  in  a  continuous  layer  of  the 
most  vivid  green.  The  tint  is  not  uniform;  long  stripes 
of  deeper  hue  alternating  with  bands  of  brighter  color. 
Close  to  the  ledge  over  which  the  water  rolls,  foam  is 
generated,  the  light  falling  upon  which,  and  flashing  back 
from  it,  is  sifted  in  its  passage  to  and  fro,  and  changed 
from  white  to  emerald- green.  Heaps  of  superficial  foam 
are  also  formed  at  intervals  along  the  ledge,  and  are  im- 
mediately drawn  into  long  white  striae.*  Lower  down,  the 
surface,   shaken   by  the  reaction  from  below,   incessantly 


*  The  direction  of  the  wind,  with  reference  to  the  course  of  a  ship,  may  be 
iaferred  with  accuracy  from  the  foam-streaks  on  the  surface  of  the  sea. 


NIAGARA  191 

rustles  into  whiteness.  The  descent  finally  resolves  itself 
into  a  rhythm,  the  water  reaching  the  bottom  of  the  fall 
in  periodic  gushes.  Nor  is  the  spray  uniformly  diffused 
through  the  air,  but  is  wafted  through  it  in  successive 
veils  of  gauze- like  texture.  From  all  this  it  is  evident 
that  beauty  is  not  absent  from  the  Horseshoe  Fall,  but 
majesty  is  its  chief  attribute.  The  plunge  of  the  water  is 
not  wild,  but  deliberate,  vast,  and  fascinating.  From  the 
Terrapin  Tower,  the  adjacent  arm  of  the  Horseshoe  is  seen 
projected  against  the  opposite  one,  midway  down;  to  the 
imagination,  therefore,  is  left  the  picturing  of  the  gulf  into 
which  the  cataract  plunges. 

The  delight  which  natural  scenery  produces  in  some 
minds  is  difficult  to  explain,  and  the  conduct  which  it 
prompts  can  hardly  be  fairly  criticised  by  those  who  have 
never  experienced  it.  It  seems  to  me  a  deduction  from 
the  completeness  of  the  celebrated  Thomas  Young,  that 
he  was  unable  to  appreciate  natural  scenery.  "He  had 
really,"  says  Dean  Peacock,  "no  taste  for  life  in  the 
country;  he  was  one  of  those  who  thought  that  no  one 
who  was  able  to  live  in  London  would  be  content  to  live 
elsewhere."  Well,  Dr.  Young,  like  Dr.  Johnson,  had  a 
right  to  his  delights;  but  I  can  understand  a  hesitation 
to   accept   them,   high   as   they  were,   to  the  exclusion  of 

That  o'erflowing  joy  which  Nature  yields 
To  her  true  lovers. 

To  all  who  are  of  this  mind,  the  strengthening  of  desire 
on  my  part  to  see  and  know  Niagara  Falls,  as  far  as  it  is 
possible  for  them  to  be  seen  and  known,  will  be  intelli- 
gible. 

On  the  first  evening  of  my  visit,   I  met,   at  the  head 


192  FRAGMENTS   OF  SCIENCE 

of  Biddle's  Stair,  the  guide  to  the  Cave  of  the  Winds. 
He  was  in  the  prime  of  manhood — large,  well  built,  firm 
and  pleasant  in  mouth  and  eye.  My  interest  in-  the  scene 
stirred  up  his,  and  made  him  communicative.  Turning 
to  a  photograph,  he  described,  by  reference  to  it,  a  feat 
which  he  had  accomplished  some  time  previously,  and 
which  had  brought  him  almost  under  the  green  water  of 
the  Horseshoe  Fall.  "Can  you  lead  me  there  to-mor- 
row?" I  asked.  He  eyed  me  inquiringly,  weighing,  per- 
haps, the  chances  of  a  man  of  light  build,  and  with  gray 
in  his  whiskers,  in  such  an  undertaking.  "I  wish,"  I 
added,  "to  see  as  much  of  the  fall  as  can  be  seen,  and 
where  you  lead  I  will  endeavor  to  follow."  His  scrutiny 
relaxed  into  a  smile,  and  he  said,  "Very  well;  I  shall  be 
ready  for  you  to-morrow." 

On  the  morrow,  accordingly,  I  came.  In  the  hut  at 
the  head  of  Biddle's  Stair  I  stripped  wholly,  and  re- 
dressed according  to  instructions — drawing  on  two  pairs 
of  woollen  pantaloons,  three  woollen  jackets,  two  pairs  of 
socks,  and  a  pair  of  felt  shoes.  Even  if  wet,  my  guide 
assured  me  that  the  clothes  would  keep  me  from  be- 
ing chilled;  and  he  was  right.  A  suit  and  hood  of  yel- 
low oilcloth  covered  all.  Most  laudable  precautions  were 
taken  by  the  young  assistant  who  helped  to  dress  me  to 
keep  the  water  out;  but  his  devices  broke  down  imme- 
diately when  severely  tested. 

•We  descended  the  stair;  the  handle  of  a  pitchfork  do- 
ing, in  my  case,  the  duty  of  an  alpenstock.  At  the  bot- 
tom, the  guide  inquired  whether  we  should  go  first  to  the 
Cave  of  the  Winds,  or  to  the  Horseshoe,  remarking  that 
the  latter  would  try  us  most.  I  decided  on  getting  the 
roughest   done   first,  and   he  turned   to   the   left   over  the 


NTAOARA  193 

Btones.  They  were  sharp  and  trying.  The  base  of  the  first 
portion  of  the  cataract  is  covered  with  huge  bowlders,  ob- 
viously the  ruins  of  the  limestone  ledge  above.  The  water 
does  not  distribute  itself  uniformly  among  these,  but  seeks 
out  channels  through  which  it  pours  torrentially.  We 
passed  some  of  these  with  wetted  feet,  but  without  diffi- 
culty. At  length  we  came  to  the  side  of  a  more  formi- 
dable current.  My  guide  walked  along  its  edge  until  he 
reached  its  least  turbulent  portion.  Halting,  he  said, 
"This  is  our  greatest  difficulty;  if  we  can  cross  here,  we 
shall  get  far  toward  the  Horseshoe.'* 

He  waded  in.  It  evidently  required  all  his  strength 
to  steady  him.  The  water  rose  above  his  loins,  and  it 
foamed  still  higher.  He  had  to  search  for  footing,  amid 
unseen  bowlders,  against  which  the  current  rose  violently. 
He  struggled  and  swayed,  but  he  struggled  successfully, 
and  finally  reached  the  shallow  water  at  the  other  side. 
Stretching  out  his  arm,  he  said  to  me,  "Now  come  on.** 
I  looked  down  the  torrent,  as  it  rushed  to  the  river  be- 
low, which  was  seething  with  the  tumult  of  the  cataract. 
De  Saussure  recommended  the  inspection  of  Alpine  dan- 
gers, with  the  view  of  making  them  familiar  to  the  eye 
before  they  are  encountered;  and  it  is  a  wholesome  custom 
in  places  of  difficulty  to  put  the  possibility  of  an  accident 
clearly  before  the  mind,  and  to  decide  beforehand  what 
ought  to  be  done  should  the  accident  occur.  Thus  wound 
up  in  the  present  instance,  I  entered  the  water.  Even 
where  it  was  not  more  than  knee- deep,  its  power  was 
manifest.  As  it  rose  around  me,  I  sought  to  split  the 
torrent  by  presenting  a  side  to  it;  but  the  insecurity  of 
the  footing  enabled  it  to  grasp  my  loins,  twist  me  fairly 

round,    and    bring   its   impetus   to   bear   upon   my    back. 

Science — Y— 9 


194  FRAGMENTS   OF  SCIENCE 

Further  struggle  was  impossible;  and  feeling  my  balance 
hopelessly  gone,  I  turned,  flung  myself  toward  the  bank 
just  quitted,  and  was  instantly,  as  expected,  swept  into 
shallower  water. 

The  oilcloth  covering  was  a  great  encumbrance ;  it  had 
been  made  for  a  much  stouter  man,  and,  standing  upright 
after  my  submersion,  my  legs  occupied  the  centre  of  two 
bags  of  water.  My  guide  exhorted  me  to  try  again. 
Prudence  was  at  my  elbow,  whispering  dissuasion;  but, 
taking  everything  into  account,  it  appeared  more  immoral 
to  retreat  than  to  proceed.  Instructed  by  the  first  mis- 
adventure, I  once  more  entered  the  stream.  Had  the 
alpenstock  been  of  iron  it  might  have  helped  me;  but, 
as  it  was,  the  tendency  of  the  water  to  sweep  it  out  of  my 
hands  rendered  it  worse  than  useless.  I,  however,  clung 
to  it  by  habit.  Again  the  torrent  rose,  and  again  I  wa- 
vered; but,  by  keeping  the  left  hip  well  against  it,  I  re- 
mained upright,  and  at  length  grasped  the  hand  of  my 
leader  at  the  other  side.  He  laughed  pleasantly.  The 
first  victory  was  gained,  and  he  enjoyed  it.  *'No  travel- 
ler," he  said,  "was  ever  here  before."  Soon  afterward, 
by  trusting  to  a  piece  of  driftwood  which  seemed  firm, 
I  was  again  taken  off  my  feet,  but  was  immediately  caught 
by  a  protruding  rock. 

We  clambered  over  the  bowlders  toward  the  thickest 
spray,  which  soon  became  so  weighty  as  to  cause  us  to 
stagger  under  its  shock.  For  the  most  part  nothing  could 
be  seen;  we  were  in  the  midst  of  bewildering  tumult, 
lashed  by  the  water,  which  sounded  at  times  like  the 
cracking  of  innumerable  whips.  Underneath  this  was 
the  deep  resonant  roar  of  the  cataract.  I  tried  to  shield 
my  eyes  with  my  hands,  and  look  upward;    but  the  de* 


NIAGARA  195 

fence  was  useless.  The  guide  continued  to  move  on,  but 
at  a  certain  place  he  halted,  desiring  me  to  take  shelter 
in  his  lee,  and  observe  the  cataract.  The  spray  did  not 
come  so  much  from  the  upper  ledge,  as  from  the  rebound 
of  the  shattered  water  when  it  struck  the  bottom.  Hence 
the  eyes  could  be  protected  from  the  blinding  shock  of 
the  spray,  while  the  line  of  vision  to  the  upper  ledges 
remained  to  some  extent  clear.  On  looking  upward  over 
the  guide's  shoulder  I  could  see  the  water  bending  over 
the  ledge,  while  the  Terrapin  Tower  loomed  fitfully 
through  the  intermittent  spray- gusts.  We  were  right  un- 
der the  tower.  A  little  further  on  the  cataract,  after  its 
first  plunge,  hit  a  protuberance  some  way  down,  and  flew 
from  it  in  a  prodigious  burst  of  spray;  through  this  we 
staggered.  We  rounded  the  promontory  on  which  the 
Terrapin  Tower  stands,  and  mov-ed,  amid  the  wildest  com- 
motion, along  the  arm  of  the  Horseshoe,  until  the  bowl- 
ders failed  us,  and  the  cataract  fell  into  the  profound 
gorge  of  the  Niagara  River. 

Here  the  guide  sheltered  me  again,  and  desired  me  to 
look  up;  I  did  so,  and  could  see,  as  before,  the  green 
gleam  of  the  mighty  curve  sweeping  over  the  upper  ledge, 
and  the  fitful  plunge  of  the  water,  as  the  spray  between 
us  and  it  alternately  gathered  and  disappeared.  An  emi- 
nent friend  of  mine  often  speaks  of  the  mistake  of  those 
physicians  who  regard  man's  ailments  as  purely  chemical, 
to  be  met  by  chemical  remedies  only.  He  contends  for 
the  psychological  element  of  cure.  By  agreeable  emo- 
tions, he  says,  nervous  currents  are  liberated  which  stimu- 
late blood,  brain,  and  viscera.  The  influence  rained  from 
ladies'  eyes  enables  my  friend  to  thrive  on  dishes  which 
would  kill  him  if  eaten  alone.     A  sanative  effect  of  the 


196  FRAGMENTS   OF  SCIENCE 

same  order  1  experienced  amid  the  spray  and  thunder  of 
Niagara.  Quickened  by  the  emotions  there  aroused,  the 
blood  sped  exultingly  through  the  arteries,  abolishing  in- 
trospection, clearing  the  heart  of  all  bitterness,  and  ena- 
bling one  to  think  with  tolerance,  if  not  with  tenderness, 
on  the  most  relentless  and  unreasonable  foe.  Apart  from 
its  scientific  value,  and  purely  as  a  moral  agent,  the  play 
was  worth  the  candle.  My  companion  knew  no  more  of 
me  than  that  I  enjoyed  the  wildness  of  the  scene;  but  as 
I  bent  in  the  shelter  of  his  large  frame  he  said,  "I  should 
like  to  see  you  attempting  to  describe  all  this."  He 
rightly  thought  it  indescribable.  The  name  of  this  gal- 
lant fellow  was  Thomas  Conroy. 

We  returned,  clambering  at  intervals  up  and  down,  so 
as  to  catch  glimpses  of  the  most  impressive  portions  of 
the  cataract.  We  passed  under  ledges  formed  by  tabular 
masses  of  limestone,  and  through  some  curious  openings 
formed  by  the  falling  together  of  the  summits  of  the  rocks. 
At  length  we  found  ourselves  beside  our  enemy  of  the 
morning.  Conroy  halted  for  a  minute  or  two,  scanning 
the  torrent  thoughtfully.  I  said  that,  as  a  guide,  he  ought 
to  have  a  rope  in  such  a  place;  but  he  retorted  that,  as 
no  traveller  had  ever  thought  of  coming  there,  he  did 
not  see  the  necessity  of  keeping  a  rope.  He  waded  in. 
The  struggle  to  keep  himself  erect  was  evident  enough; 
he  swayed,  but  recovered  himself  again  and  again.  At 
length  he  slipped,  gave  way,  did  as  I  had  done,  threw 
himself  toward  the  bank,  and  was  swept  into  the  shal- 
lows. Standing  in  the  stream  near  its  edge,  he  stretched 
his  arm  toward  me.  I  retained  the  pitchfork  handle,  for 
it  had  been  useful  among  the  bowlders.  By  wading  some 
way  in,  the  stafE  could  be  made  to  reach  him,  and  I  pro- 


NIAGARA  197 

posed  his  seizing  it.  *'If  you  are  sure,'*  he  replied,  *'that, 
in  case  of  giving  way,  you  can  maintain  your  grasp,  then 
I  will  certainly  hold  you."  Kemarking  that  he  might 
count  on  this,  I  waded  in,  and  stretched  the  staff  to  my 
companion.  It  was  firmly  grasped  by  both  of  us.  Thus 
helped,  though  its  onset  was  strong,  I  moved  safely  across 
the  torrent.  All  danger  ended  here.  We  afterward 
roamed  sociably  among  the  torrents  and  bowlders  below 
the  Cave  of  the  Winds.  The  rocks  were  covered  with 
organic  slime,  which  could  not  have  been  walked  over 
with  bare  feet,  but  the  felt  shoes  effectually  prevented 
slipping.  We  reached  the  cave  and  entered  it,  first  by 
a  wooden  way  carried  over  the  bowlders,  and  then  along  a 
narrow  ledge,  to  the  point  eaten  deepest  into  the  shale. 
When  the  wind  is  from  the  south,  the  falling  water, 
I  am  told,  can  be  seen  tranquilly  from  this  spot;  but 
when  we  were  there,  a  blinding  hurricane  of  spray 
was  whirled  against  us.  On  the  evening  of  the  same 
day,  I  went  behind  the  water  on  the  Canada  side,  which, 
after  the  experiences  of  the  morning,  struck  me  as  an 
imposture. 

Still  even  this  latter  is  exciting  to  some  nerves.  Its 
effect  upon  himself  is  thus  vividly  described  by  Mr.  Bake- 
well,  Jr. :  *^0n  turning  a  sharp  angle  of  the  rock,  a  sudden 
gust  of  wind  met  us,  coming  from  the  hollow  between  the 
fall  and  the  rock,  which  drove  the  spray  directly  in  our 
faces,  with  such  force  that  in  an  instant  we  were  wet 
through.  When  in  the  midst  of  this  shower-bath  the  shock 
took  away  my  breath:  I  turned  back  and  scrambled  over 
the  loose  stones  to  escape  the  conflict.  The  guide  soon 
followed,  and  told  me  that  I  had  passed  the  worst  part. 
With  that  assurance  I  made  a  second    attempt;   but  so 


198  FRAGMENTS    OF  SCIENCE 

wild  and  disordered  was  my  imagination  that  when  I  had 
reached  half  way  I  could  bear  it  no  longer. ' '  ' 

To  complete  my  knowledge  I  desired  to  see  the  fall 
from  the  river  below  it,  and  long  negotiations  were  neces- 
sary to  secure  the  means  of  doing  so.  The  only  boat  fit 
for  the  undertaking  had  been  laid  up  for  the  winter;  but 
this  difhculty,  through  the  kind  intervention  of  Mr.  Town- 
send,  was  overcome.  The  main  one  was  to  secure  oarsmen 
sufficiently  strong  and  skilful  to  urge  the  boat  where  I 
wished  it  to  be  taken.  The  son  of  the  owner  of  the  boat, 
a  finely-built  young  fellow,  but  only  twenty,  and  therefore 
not  sufficiently  hardened,  was  willing  to  go;  and  up  the 
river,  it  was  stated,  there  lived  another  man  who  could  do 
anything  with  the  boat  which  strength  and  daring  could 
accomplish.  He  came.  His  figure  and  expression  of  face 
certainly  indicated  extraordinary  firmness  and  power.  On 
Tuesday,  November  5,  we  started,  each  of  us  being  clad 
in  oilcloth.  The  elder  oarsman  at  once  assumed  a  tone  of 
authority  over  his  companion,  and  struck  immediately  in 
amid  the  breakers  below  the  American  Fall.  He  hugged 
the  cross  freshets  instead  of  striking  out  into  the  smoother 
water.  I  asked  him  why  he  did  so,  and  he  replied  that 
they  were  directed  outward,  not  downward.  The  struggle, 
however,  to  prevent  the  bow  of  the  boat  from  being  turned 
by  them  was  often  very  severe. 

The  spray  was  in  general  blinding,  but  at  times  it  dis- 
appeared and  yielded  noble  views  of  the  fall.  The  edge 
of  the  cataract  is  crimped  by  indentations  which  exalt  its 
beauty.  Here  and  there,  a  little  below  the  highest/  ledge, 
a  secondary  one  juts  out;  the  water  strikes  it  and  bursts 

>  "Mag.  of  Nat.  Hist.,"  1830,  pp.  121,  122. 


NIAGARA  199 

from  it  in  huge  protuberant  masses  of  foam  and  spray. 
We  passed  Goat  Island,  came  to  the  Horseshoe,  and 
worked  for  a  time  along  its  base,  the  bowlders  over  which 
Conroy  and  myself  had  scrambled  a  few  days  previously 
lying  between  us  and  the  cataract.  A  rock  was  before  us, 
concealed  and  revealed  at  intervals,  as  the  waves  passed 
over  it.  Our  leader  tried  to  get  above  this  rock,  first  on 
the  outside  of  it.  The  water,  however,  was  here  in  vio- 
lent motion.  The  men  struggled  fiercely,  the  older  one 
ringing  out  an  incessant  peal  of  command  and  exhortation 
to  the  younger.  As  we  were  just  clearing  the  rock,  the 
bow  came  obliquely  to  the  surge;  the  boat  was  turned 
suddenly  round  and  shot  with  astonishing  rapidity  down 
the  river.  The  men  returned  to  the  charge,  now  trying  to 
get  up  between  the  half -concealed  rock  and  the  bowlders 
to  the  left.  But  the  torrent  set  in  strongly  through  this 
channel.  The  tugging  was  quick  and  violent,  but  we 
made  little  way.  At  length,  seizing  a  rope,  the  princi- 
pal oarsman  made  a  desperate  attempt  to  get  upon  one  of 
the  bowlders,  hoping  to  be  able  to  drag  the  boat  through 
the  channel;  but  it  bumped  so  violently  against  the  rock 
that  the  man  flung  himself  back  and  relinquished  the 
attempt. 

We  returned  along  the  base  of  the  American  Fall,  run- 
ning in  and  out  among  the  currents  which  rushed  from  it 
laterally  into  the  river.  Seen  from  below,  the  American 
Fall  is  certainly  exquisitely  beautiful,  but  it  is  a  mere  frill 
of  adornment  to  its  nobler  neighbor  the  Horseshoe.  At 
times  we  took  to  the  river,  from  the  centre  of  which  the 
Horseshoe  Fall  appeared  especially  magnificent.  A  streak 
of  cloud  across  the  neck  of  Mont  Blanc  can  double  its 
apparent  height,  so  here  the  green  summit  of  the  cataract 


200  FRAGMENTS   OF  SCIENCE 

stining  above  tlie  smoke  of  spray  appeared  lifted  to  an 
extraordinary  elevation.  Had  Hennepin  and  La  Hontan 
seen  the  fall  from  this  position,  their  estimates  of  the 
height  would  have  been  perfectly  excusable. 

From  a  point  a  little  way  below  the  American  Fall,  a 
ferry  crosses  the  river,  in  summer,  to  the  Canadian  side. 
Below  the  ferry  is  a  suspension  bridge  for  carriages  and 
foot-passengers,  and  a  mile  or  two  lower  down  is  the  rail- 
way suspension  bridge.  Between  ferry  and  bridge  the 
river  Niagara  flows  unruffled;  but  at  the  suspension 
bridge  the  bed  steepens  and  the  river  quickens  its  mo- 
tion. Lower  down  the  gorge  narrows,  and  the  rapidity 
and  turbulence  increase.  At  the  place  called  the  * '  Whirl- 
pool Eapids,"  I  estimated  the  width  of  the  river  at  300 
feet,  an  estimate  confirmed  by  the  dwellers  on  the  spot. 
When  it  is  remembered  that  the  drainage  of  nearly  half 
a  continent  is  compressed  into  this  space,  the  impetuosity 
of  the  river's  rush  may  be  imagined.  Had  it  not  been  for 
Mr.  Bierstadt,  the  distinguished  photographer  of  Niagara, 
I  should  have  quitted  the  place  without  seeing  these  rap- 
ids; for  this,  and  for  his  agreeable  company  to  the  spot, 
I  have  to  thank  him.  From  the  edge  of  the  cliff  above 
the  rapids  we  descended — a  little,  I  confess,  to  a  climber's 
disgust — in  an  "elevator,"  because  the  effects  are  best  seen 
from  the  water  level. 

Two  kinds  of  motion  are  here  obviously  active,  a  mo- 
tion of  translation  and  a  motion  of  undulation — the  race 
of  the  river  through  its  gorge,  and  the  great  waves  gen- 
erated by  its  collision  with,  and  rebound  from,  the  obsta- 
cles in  its  way.  In  the  middle  of  the  river  the  rush  and 
tossing  are  most  violent  j  at  all  events,  the  impetuous  force 


NIAGARA  201 

of  the  individual  waves  is  here  most  strikingly  displayed. 
Vast  pyramidal  heaps  leap  incessantly  from  the  river,  some 
of  them  with  such  energy  as  to  jerk  their  summits  into  the 
air,  where  they  hang  momentarily  suspended  in  crowds  of 
liquid  spherules.  The  sun  shone  for  a  few  minutes.  At 
times  the  wind,  coming  up  the  river,  searched  and  sifted  the 
spray,  carrying  away  the  lighter  drops  and  leaving  the  heav- 
ier ones  behind.  Wafted  in  the  proper  direction,  rainbows 
appeared  and  disappeared  fitfully  in  the  lighter  mist.  In 
other  directions  the  common  gleam  of  the  sunshine  from 
the  waves  and  their  shattered  crests  was  exquisitely  beau- 
tiful. The  complexity  of  the  action  was  still  further  illus- 
trated by  the  fact,  that  in  some  cases,  as  if  by  the  exercise 
of  a  local  explosive  force,  the  drops  were  shot  radially  from 
a  particular  centre,  forming  around  it  a  kind  of  halo. 

The  first  impression,  and,  indeed,  the  current  explana- 
tion, of  these  rapids  is,  that  the  central  bed  of  the  river 
is  cumbered  with  large  bowlders,  and  that  the  jostling, 
tossing  and  wild  leaping  of  the  water  there  are  due  to  its 
impact  against  these  obstacles.  I  doubt  this  explanation. 
At  all  events,  there  is  another  sufiicient  reason  to  be  taken 
into  account.  Bowlders  derived  from  the  adjacent  cliffs 
visibly  cumber  the  sides  of  the  river.  Against  these  the 
water  rises  and  sinks  rhythmically  but  violently,  large 
waves  being  thus  produced.  On  the  generation  of  each 
wave,  there  is  an  immediate  compounding  of.  the  wave- 
motion  with  the  river-motion.  The  ridges,  which  in  still 
water  would  proceed  in  circular  curves  round  the  centre 
of  disturbance,  cross  the  river  obliquely,  and  the  result 
is  that  at  the  centre  waves  commingle  which  have  really 
been  generated  at  the  sides.  In  the  first  instance,  we  had 
a  composition  of  wave-motion  with  river-motion;  here  we 


202  FRAGMENTS   OF  SCIENCE 

have  tlie  coalescence  of  waves  with  waves.  Where  crest 
and  furrow  cross  each  other,  the  motion  is  annulled ;  where 
furrow  and  furrow  cross,  the  river  is  plowed  to  a  greater 
depth;  and  where  crest  and  crest  aid  each  other,  we  have 
that  astonishing  leap  of  the  water  which  breaks  the  cohe- 
sion of  the  crests,  and  tosses  them  shattered  into  the  air. 
From  the  water  level  the  cause  of  the  action  is  not  so 
easily  seen;  but  from  the  summit  of  the  cliff  the  lateral 
generation  of  the  waves,  and  their  propagation  to  the  cen- 
tre, are  perfectly  obvious.  If  this  explanation  be  correct, 
the  phenomena  observed  at  the  Whirlpool  Eapids  form  one 
of  the  grandest  illustrations  of  the  principle  of  interference. 
The  Nile  "cataract,"  Mr.  Huxley  informs  me,  offers  more 
moderate  examples  of  the  same  action. 

At  some  distance  below  the  Whirlpool  Eapids  we  have 
the  celebrated  whirlpool  itself.  Here  the  river  makes  a 
sudden  bend  to  the  northeast,  forming  nearly  a  right  angle 
with  its  previous  direction.  The  water  strikes  the  concave 
bank  with  great  force,  and  scoops  it  incessantly  away.  A 
vast  basin  has  been  thus  formed,  in  which  the  sweep  of 
the  river  prolongs  itself  in  gyratory  currents.  Bodies  and 
trees  which  have  come  over  the  falls  are  stated  to  circulate 
here  for  days  without  finding  the  outlet.  From  various 
points  of  the  cliffs  above  this  is  curiously  hidden.  The 
rush  of  the  river  into  the  whirlpool  is  obvious  enough ;  and 
though  you  imagine  the  outlet  must  be  visible,  if  one  existed, 
you  cannot  find  it.  Turning,  however,  round  the  bend  of 
the  precipice  to  the  northeast,  the  outlet  comes  into  view. 

The  Niagara  season  was  over;  the  chatter  of  sight- seers 
had  ceased,  and  the  scene  presented  itself  as  one  of  holy 
seclusion  and  beauty.  I  went  down  to  the  river's  edge, 
where  the  weird  loneliness  seemed  to  increase.     The  basin 


NIAGARA  203 

is  enclosed  by  high  and  almost  precipitous  banks — covered, 
at  the  time,  with  russet  woods.  A  kind  of  mystery  at- 
taches itself  to  gyrating  water,  due  perhaps  to  the  fact 
that  we  are  to  some  extent  ignorant  of  the  direction  of 
its  force.  It  is  said  that,  at  certain  points  of  the  whirl- 
pool, pine-trees  are  sucked  down,  to  be  ejected  mysteri- 
ously elsewhere.  The  water  is  of  the  brightest  emerald- 
green.  The  gorge  through  which  it  escapes  is  narrow, 
and  the  motion  of  the  river  swift  though  silent.  The 
surface  is  steeply  inclined,  but  it  is  perfectly  unbroken. 
There  are  no  lateral  waves,  no  ripples  with  their  breaking 
bubbles  to  raise  a  murmur;  while  the  depth  is  here  too  great 
to  allow  the  inequality  of  the  bed  to  ruffle  the  surface. 
Nothing  can  be  more  beautiful  than  this  sloping  liquid 
mirror  formed  by  the  Niagara  in  sliding  from  the  whirlpool. 
The  green  color  is,  I  think,  correctly  accounted  for 
in  the  last  Fragment.  While  crossing  the  Atlantic,  in 
1872-1873,  I  had  frequent  opportunities  of  testing  the 
explanation  there  given.  Looked  properly  down  upon, 
there  are  portions  of  the  ocean  to  which  we  should  hardly 
ascribe  a  trace  of  blue;  at  the  most,  a  mere  hint  of  indigo 
reaches  the  eye.  The  water,  indeed,  is  practically  black, 
and  this  is  an  indication  both  of  its  depth  and  of  its  free- 
dom from  mechanically  suspended  matter.  In  small  thick- 
nesses water  is  sensibly  transparent  to  all  kinds  of  light; 
but,  as  the  thickness  increases,  the  rays  of  low  refrangi- 
bility  are  first  absorbed,  and  after  them  the  other  rays. 
"Where,  therefore,  the  water  is  very  deep  and  very  pure, 
all  the  colors  are  absorbed,  and  such  water  ought  to  ap- 
pear black,  as  no  light  is  sent  from  its  interior  to  the  eye. 
The  approximation  of  the  Atlantic  Ocean  to  this  condition 
is  an  indication  of  its  extreme  purity. 


204  FRAGMENTS   OF  SCIENCE 

Throw  a  white  pebble  into  such  water;  as  it  sinks  it 
becomes  greener  and  greener,  and,  before  it  disappears, 
it  reaches  a  vivid  blue-gi"een.  Break  such  a  pebble  into 
fragments,  each  of  these  will  behave  like  the  unbroken 
mass;  grind  the  pebble  to  powder,  every  particle  will 
yield  its  modicum  of  green;  and  if  the  particles  be  so  fine 
as  to  remain  suspended  in  the  water,  the  scattered  light 
will  be  a  uniform  green.  Hence  the  greenness  of  shoal 
water.  You  go  to  bed  with  the  black  Atlantic  around 
you.  You  rise  in  the  morning,  find  it  a  vivid  green,  and 
correctly  infer  that  you  are  crossing  the  bank  of  New- 
foundland. Such  water  is  found  charged  with  fine  matter 
in  a  state  of  mechanical  suspension.  The  light  from  the 
bottom  may  sometimes  come  into  play,  but  it  is  not  neces- 
sary. A  storm  can  render  the  water  muddy,  by  render- 
ing the  particles  too  numerous  and  gross.  Such  a  case 
occurred  toward  the  close  of  my  visit  to  Niagara.  There 
had  been  rain  and  storm  in  the  upper  lake- regions,  and 
the  quantity  of  suspended  matter  brought  down  quite 
extinguished  the  fascinating  green  of  the  Horseshoe. 

Nothing  can  be  more  superb  than  the  green  of  the 
Atlantic  waves,  when  the  circumstances  are  favorable  to 
the  exhibition  of  the  color.  As  long  as  a  wave  remains 
unbroken  no  color  appears;  but  when  the  foam  just 
doubles  over  the  crest,  like  an  Alpine  snow-cornice,  un- 
der the  cornice  we  often  see  a  display  of  the  most  exqui- 
site green.  It  is  metallic  in  its  brilliancy.  But  the  foam 
is  necessary  to  its  production.  The  foam  is  first  illumi- 
nated, and  it  scatters  the  light  in  all  directions;  the  light 
which  passes  through  the  higher  portion  of  the  wave  alone 
reaches  the  eye,  and  gives  to  that  portion  its  matchless 
color.     The  folding  of  the  wave,  producing  as  it  does  a 


NIAGARA  205 

series  of  longitudinal  protuberances  and  furrows  which 
act  like  cylindrical  lenses,  introduces  variations  in  the 
intensity  of  the  light,  and  materially  enhances  its  beauty. 

We  have  now  to  consider  the  genesis  and  proximate 
destiny  of  the  Falls  of  Niagara.  We  may  open  our  way 
to  this  subject  by  a  few  preliminary  remarks  upon  erosion. 
Time  and  intensity  are  the  main  factors  of  geologic  change, 
and  they  are  in  a  certain  sense  convertible.  A  feeble 
force  acting  through  long  periods,  and  an  intense  force 
acting  through  short  ones,  may  produce  approximately 
the  same  results.  To  Dr.  Hooker  I  have  been  indebted 
for  some  specimens  of  stones,  the  first  examples  of  which 
were  picked  up  by  Mr.  Hackworth  on  the  shores  of 
Lyell's  Bay,  near  Wellington,  in  New  Zealand.  They 
were  described  by  Mr.  Travers  in  the  "Transactions  of 
the  New  Zealand  Institute."  Unacquainted  with  their 
origin,  you  would  certainly  ascribe  their  forms  to  human 
workmanship.  They  resemble  knives  and  spear-heads, 
being  apparently  chiselled  off  into  facets,  with  as  much 
attention  to  symmetry  as  if  a  tool,  guided  by  human 
intelligence,  had  passed  over  them.  But  no  human  in- 
strument has  been  brought  to  bear  upon  these  stones. 
They  have  been  wrought  into  their  present  shape  by  the 
wind-blown  sand  of  Lyell's  Bay.  Two  winds  are  domi- 
nant here,  and  they  in  succession  urged  the  sand  against 
opposite  sides  of  the  stone;  every  little  particle  of  sand 
chipped  away  its  infinitesimal  bit  of  stone,  and  in  the  end 
sculptured  these  singular  forms.  * 

*  **These  stones,  which  have  a  strong  resemblance  to  works  of  human  art, 
occur  in  great  abundance,  and  of  various  sizes,  from  half  an  inch  to  several 
inches  in  length.     A  large  number  were  exhibited  showing  the  various  forms, 


206  FRAGMENTS   OF  SCIENCE 

The  Sphinx  of  Egypt  is  nearly  covered  up  by  the 
sand  of  the  desert.  The  neck  of  the  Sphinx  is  partly  cut 
across,  not,  as  I  am  assured  by  Mr.  Huxley,  by  ordinary 
weathering,  but  by  the  eroding  action  of  the  fine  sand 
blown  against  it.  In  these  cases  Nature  furnishes  us  with 
hints  which  may  be  taken  advantage  of  in  art;  and  this 
action  of  sand  has  been  recently  turned  to  extraordinary 
account  in  the  United  States.  When  in  Boston,  I  was 
taken  by  my  courteous  and  helpful  friend,  Mr.  Josiah 
Quincy,  to  see  the  action  of  the  sand-blast.  A  kind  of 
hopper  containing  fine  siliceous  sand  was  connected  with 
a  reservoir  of  compressed  air,  the  pressure  being  variable 
at  pleasure.  The  hopper  ended  in  a  long  slit,  from  which 
the  sand  was  blown.  A  plate  of  glass  was  placed  beneath 
this  slit,  and  caused  to  pass  slowly  under  it;  it  came  out 
perfectly  depolished,  with  a  bright  opalescent  glimmer, 
such  as  could  only  be  produced  by  the  most  careful  grind- 
ing. Every  little  particle  of  sand  urged  against  the  glass, 
having  all  its  energy  concentrated  on  the  point  of  impact, 
formed  there  a  little  pit,  the  depolished  surface  consisting 
of  innumerable  hollows  of  this  description. 

which  are  those  of  wedges,  knives,  arrow-heads,  etc.,  and  all  with  sharp 
cutting  edges. 

"Mr.  Travers  explained  that,  notwithstanding  their  artificial  appearance, 
these  stones  were  formed  by  the  cutting  action  of  the  wind- driven  sand,  as  it 
passed  to  and  fro  over  an  exposed  bowlder- bank.  He  gave  a  minute  account 
of  the  manner  in  which  the  varieties  of  form  are  produced,  and  referred  to  the 
effect  which  the  erosive  action  thus  indicated  would  have  on  railway  and  other 
works  executed  on  sandy  tracts. 

"Dr.  Hector  stated  that  although,  as  a  group,  the  specimens  on  the  table 
could  not  well  be  mistaken  for  artificial  productions,  still  the  forms  are  so 
peculiar,  and  the  edges,  in  a  few  of  them,  so  perfect,  that  if  they  were  discov- 
ered associated  with  human  works  there  is  no  doubt  that  they  would  have  been 
referred  to  the  so-called  'stone  period.'" — Extracted  from  the  Minutes  of  the 
Wellington  Philosophical  Society,  February  9,  1869. 


NIAGARA  207 

But  this  was  not  all.  By  protecting  certain  portions 
of  the  surface,  and  exposing  others,  figures  and  tracery  of 
any  required  form  could  be  etched  upon  the  glass.  The 
figures  of  open  iron- work  could  be  thus  copied;  while 
wire- gauze  placed  over  the  glass  produced  a  reticulated 
pattern.  But  it  required  no  such  resisting  substance  as 
iron  to  shelter  the  glass.  The  patterns  of  the  finest  lace 
could  be  thus  reproduced;  the  delicate  filaments  of  the 
lace  itself  offering  a  sufficient  protection.  All  these 
effects  have  been  obtained  with  a  simple  model  of  the 
sand-blast  devised  by  my  assistant.  A  fraction  of  a 
minute  suffices  to  etch  upon  glass  a  rich  and  beautiful 
lace  pattern.  Any  yielding  substance  may  be  employed 
to  protect  the  glass.  By  diffusing  the  shock  of  the  par- 
ticle, such  substances  practically  destroy  the  local  erosive 
power.  The  hand  can  bear,  without  inconvenience,  a 
sand- shower  which  would  pulverize  glass.  Etchings  exe- 
cuted on  glass  with  suitable  kinds  of  ink  are  accurately 
worked  out  by  the  sand-blast.  In  fact,  within  certain 
limits,  the  harder  the  surface,  the  greater  is  the  concen- 
tration of  the  shock,  and  the  more  effectual  is  the  erosion. 
It  is  not  necessary  that  the  sand  should  be  the  harder 
substance  of  the  two;  corundum,  for  example,  is  much 
harder  than  quartz;  still,  quartz-sand  can  not  only  depol- 
ish,  but  actually  blow  a  hole  through  a  plate  of  corundum. 
Nay,  glass  may  be  depolished  by  the  impact  of  fine  shot; 
the  grains  in  this  case  bruising  the  glass  before  they  have 
time  to  flatten  and  turn  their  energy  into  heat. 

And  here,  in  passing,  we  may  tie  together  one  or  two 
apparently  unrelated  facts.  Supposing  you  turn  on,  at 
the  lower  part  of  a  house,  a  cock  which  is  fed  by  a  pipe 
from   a  cistern   at  the   top   of  the  house,    the   column  of 


208  FRAGMENTS   OF  SCIENCE 

water,  from  the  cistern  downward,  is  set  in  motion.  By 
turning  off  the  cock,  this  motion  is  stopped;  and  when 
the  turning  off  is  very  sudden,  the  pipe,  if  not  strong, 
may  be  burst  by  the  internal  impact  of  the  water.  By 
distributing  the  turning  of  the  cock  over  half  a  second  of 
time,  the  shock  and  danger  of  rupture  may  be  entirely 
avoided.  "We  have  here  an  example  of  the  concentration 
of  energy  in  time.  The  sand-blast  illustrates  the  concen- 
tration of  energy  in  space.  The  action  of  flint  and  steel 
is  an  illustration  of  the  same  principle.  The  heat  re- 
quired to  generate  the  spark  is  intense;  and  the  mechan- 
ical action,  being  moderate,  must,  to  produce  fire,  be  in 
the  highest  degree  concentrated.  This  concentration  is 
secured  by  the  collision  of  hard  substances.  Calc-spar 
will  not  supply  the  place  of  flint,  nor  lead  the  place  of 
steel,  in  the  production  of  fire  by  collision.  With  the 
softer  substances,  the  total  heat  produced  may  be  greater 
than  with  the  hard  ones,  but,  to  produce  the  spark,  the 
heat  must  be  intensely  localized. 

We  can,  however,  go  far  beyond  the  mere  depolishing 
of  glass;  indeed  I  have  already  said  that  quartz-sand  can 
wear  a  hole  through  corundum.  This  leads  me  to  express 
my  acknowledgments  to  General  Tilghman,*  who  is  the 
inventor  of  the  sand-blast.  To  his  spontaneous  kindness 
I  am  indebted  for  some  beautiful  illustrations  of  his  proc- 


*  The  absorbent  power,  if  I  may  use  the  phrase,  exerted  by  the  industrial 
arts  in  the  United  States,  is  forcibly  illustrated  by  the  rapid  transfer  of  men  like 
Mr.  Tilghman  from  the  life  of  the  soldier  to  that  of  the  civilian.  General 
McClelian,  now  a  civil  engineer,  whom  I  had  the  honor  of  frequently  meeting 
in  New  York,  is  a  most  eminent  example  of  the  same  kind.  At  the  end  of  the 
war,  indeed,  a  million  and  a  half  of  men  were  thus  drawn,  in  an  astonishingly 
short  time,  from  military  to  civil  life. 


NIAGARA  209 

ess.  In  one  thick  plate  of  glass  a  figure  lias  been  worked 
out  to  a  depth  of  fths  of  an  inch.  A  second  plate,  Jths 
of  an  inch  thick,  is  entirely  perforated.  In  a  circular 
plate  of  marble,  nearly  half  an  inch  thick,  open  work  of 
most  intricate  and  elaborate  description  has  been  exe- 
cuted. It  would  probably  take  many  days  to  perform  this 
work  by  any  ordinary  process;  with  the  sand-blast  it  was 
accomplished  in  an  hour.  So  much  for  the  strength  of 
the  blast;  its  delicacy  is  illustrated  by  this  beautiful  ex- 
ample of  line  engraving,  etched  on  glass  by  means  of  the 
blast. 

This  power  of  erosion,  so  strikingly  displayed  when 
sand  is  urged  by  air,  renders  us  better  able  to  conceive 
its  action  when  urged  by  water.  The  erosive  power  of 
a  river  is  vastly  augmented  by  the  solid  matter  carried 
along  with  it.  Sand  or  pebbles,  caught  in  a  river  vor- 
tex, can  wear  away  the  hardest  rock;  ** potholes'*  and 
deep  cylindrical  shafts  being  thus  produced.  An  ex- 
traordinary instance  of  this  kind  of  erosion  is  to  be  seen 
in  the  Yal  Tournanche,  above  the  village  of  this  name. 
The  gorge  at  Handeck  has  been  thus  cut  out.  Such 
waterfalls  were  once  frequent  in  the  valleys  of  Switzer- 
land; for  hardly  any  valley  is  without  one  or  more  trans- 
verse barriers  of  resisting  material,  over  which  the  river 
flowing  through  the  valley  once  fell  as  a  cataract.  Near 
Pontresina,  in  the  Engadin,  there  is  such  a  case;  a  hard 
gneiss  being  there  worn  away  to  form  a  gorge,  through 
which  the  river  from  the  Morteratsch  glacier  rushes.  The 
barrier  of  the  Kirchet  above  Meyringen  is  also  a  case  in 
point.  Behind  it  was  a  lake,  derived  from  the  glacier  of 
the  Aar,  and  over  the  barrier  the  lake  poured  its  excess 
of  water.     Here  the  rock,    being  limestone,    was  in  part 


210  FRAGMENTS   OF  SCIENCE 

dissolved;  but  added  to  this  we  had  the  action  of  the 
sand  and  gravel  carried  along  by  the  water,  which,  on 
striking  the  rock,  chipped  it  away  like  the  particles  of  the 
sand-blast.  Thus,  by  solution  and  mechanical  erosion, 
the  great  chasm  of  the  Finsteraarschlucht  was  formed.  It 
is  demonstrable  that  the  water  which  flows  at  the  bottoms 
of  such  deep  fissures  once  flowed  at  the  level  of  their 
present  edges,  and  tumbled  down  the  lower  faces  of  the 
barriers.  Almost  every  valley  in  Switzerland  furnishes 
examples  of  this  kind;  the  untenable  hypothesis  of  earth- 
quakes, once  so  readily  resorted  to  in  accounting  for  these 
gorges,  being  now  for  the  most  part  abandoned.  To  pro- 
duce the  cafions  of  Western  America,  no  other  cause  is 
needed  than  the  integration  of  effects  individually  infini- 
tesimal. 

And  now  we  come  to  Niagara.  Soon  after  Europeans 
had  taken  possession  of  the  country,  the  conviction  ap- 
pears to  have  arisen  that  the  deep  channel  of  the  river 
Niagara  below  the  falls  had  been  excavated  by  the  cata- 
ract. In  Mr.  Bakewell's  "Introduction  to  Geology,"  the 
prevalence  of  this  belief  has  been  referred  to.  It  is  ex- 
pressed thus  by  Professor  Joseph  Henry  in  the  "Transac- 
tions of  the  Albany  Institute" :  '  "In  viewing  the  position 
of  the  falls,  and  the  features  of  the  country  round,  it  is 
impossible  not  to  be  impressed  with  the  idea  that  this  great 
natural  raceway  has  been  formed  by  the  continued  action 
of  the  irresistible  Niagara,  and  that  the  falls,  beginning  at 
Lewiston,  have,  in  the  course  of  ages,  worn  back  the  rocky 
strata  to  their  present  site."  The  same  view  is  advocated 
by  Sir  Charles  Lyell,  by  Mr.  Hall,  by  M.  Agassiz,  by  Pro- 

*  Quoted  by  Bakewell. 


NIAGARA  211 

fessor  Ramsay,  indeed  by  most  of  those  who  have  inspected 
the  place. 

A  connected  image  of  the  origin  and  progress  of  the 
cataract  is  easily  obtained.  Walking  northward  from  the 
village  of  Niagara  Falls  by  the  side  of  the  river,  we  have 
to  our  left  the  deep  and  comparatively  narrow  gorge, 
through  which  the  Niagara  flows.  The  bounding  clifis  of 
this  gorge  are  from  800  to  850  feet  high.  We  reach  the 
whirlpool,  trend  to  the  northeast,  and  after  a  little  time 
gradually  resume  our  northward  course.  Finally,  at  about 
seven  miles  from  the  present  falls,  we  come  to  the  edge  of 
a  declivity,  which  informs  us  that  we  have  been  hitherto 
walking  on  table-land.  At  some  hundreds  of  feet  below 
us  is  a  comparatively  level  plain,  which  stretches  to  Lake 
Ontario.  The  declivity  marks  the  end  of  the  precipitous 
gorge  of  the  Niagara.  Here  the  river  escapes  from  its 
steep  mural  boundaries,  and  in  a  widened  bed  pursues 
its  way  to  the  lake  which  finally  receives  its  waters. 

The  fact  that  in  historic  times,  even  within  the  mem- 
ory of  man,  the  fall  has  sensibly  receded,  prompts  the 
question,  How  far  has  this  recession  gone  ?  At  what  point 
did  the  ledge  which  thus  continually  creeps  backward  be- 
gin its  retrograde  course?  To  minds  disciplined  in  such 
researches  the  answer  has  been,  and  will  be — At  the  pre- 
cipitous declivity  which  crossed  the  Niagara  from  Lewis- 
ton  on  the  American  to  Queenston  on  the  Canadian  side. 
Over  this  transverse  barrier  the  united  affluents  of  all  the 
upper  lakes  once  poured  their  waters,  and  here  the  work 
of  erosion  began.  The  dam,  moreover,  was  demonstrably 
of  sufficient  height  to  cause  the  river  above  it  to  submerge 
Goat  Island;  and  this  would  perfectly  account  for  the  find- 
ing, by  Sir  Charles  Lyell,  Mr.   Hall,  and  others,  in  the 


212  FRAGMENTS   OF  SCIENCE 

sand  and  gravel  of  the  island,  the  same  flnviatile  shells 
as  are  now  found  in  the  Niagara  Kiver  higher  up.  It 
would  also  account  for  those  deposits  along  the  sides  of 
the  river,  the  discovery  of  which  enabled  Lyell,  Hall,  and 
Eamsay  to  reduce  to  demonstration  the  popular  belief  that 
the  Niagara  once  flowed  through  a  shallow  valley. 

The  physics  of  the  problem  of  excavation,  which  I  made 
clear  to  my  mind  before  quitting  Niagara,  are  revealed  by 
a  close  inspection  of  the  present  Horseshoe  Fall.  We  see 
evidently  that  the  greatest  weight  of  water  bends  over  the 
very  apex  of  the  Horseshoe.  In  a  passage  in  his  excellent 
chapter  on  Niagara  Falls,  Mr.  Hall  alludes  to  this  fact. 
Here  we  have  the  most  copious  and  the  most  violent  whirl- 
ing of  the  shattered  liquid;  here  the  most  powerful  eddies 
recoil  against  the  shale.  From  this  portion  of  the  fall,  in- 
deed, the  spray  sometimes  rises  without  solution  of  conti- 
nuity to  the  region  of  clouds,  becoming  gradually  more  at- 
tenuated, and  passing  finally  through  the  condition  of  true 
cloud  into  invisible  vapor,  which  is  sometimes  reprecipitated 
higher  up.  All  the  phenomena  point  distinctly  to  the  cen- 
tre of  the  river  as  the  place  of  greatest  mechanical  energy, 
and  from  the  centre  the  vigor  of  the  fall  gradually  dies 
away  toward  the  sides.  The  Horseshoe  form,  with  the 
concavity  facing  downward,  is  an  obvious  and  necessary- 
consequence  of  this  action.  Eight  along  the  middle  of  the 
river  the  apex  of  the  curve  pushes  its  way  backward,  cut- 
ting along  the  centre  a  deep  and  comparatively  narrow 
groove,  and  draining  the  sides  as  it  passes  them.*  Hence 
the   remarkable   discrepancy   between    the  widths    of    the 


*  In  the  discourse  the  excavation  of  the  centre  and  drainage  of  the  sides 
action  was  illustrated  by  a  model  devised  by  my  assistant,  Mr.  John  Cottrell. 


NIAGARA  216 

Niagara  above  and  below  the  Horseshoe.  All  along  its 
course,  from  Lewiston  Heights  to  its  present  position,  the 
form  of  the  fall  was  probably  that  of  a  horseshoe;  for  this 
is  merely  the  expression  of  the  greater  depth,  and  conse- 
quently greater  excavating  power,  of  the  centre  of  the 
river.  The  gorge,  moreover,  varies  in  width,  as  the  depth 
of  the  centre  of  the  ancient  river  varied,  being  narrowest 
where  that  depth  was  greatest. 

The  vast  comparative  erosive  energy  of  the  Horseshoe 
Fall  comes  strikingly  into  view  when  it  and  the  American 
Fall  are  compared  together.  The  American  branch  of  the 
river  is  cut  at  a  right  angle  by  the  gorge  of  the  Niagara. 
Here  the  Horseshoe  Fall  was  the  real  excavator.  It  cut 
the  rock,  and  formed  the  precipice,  over  which  the  Amer- 
ican Fall  tumbles.  But,  since  its  formation,  the  erosive 
action  of  the  American  Fall  has  been  almost  nil,  while  the 
Horseshoe  has  cut  its  way  for  600  yards  across  the  end 
of  Goat  Island,  and  is  now  doubling  back  to  excavate  its 
channel  parallel  to  the  length  of  the  island.  This  point, 
which  impressed  me  forcibly,  has  not,  I  have  just  learned, 
escaped  the  acute  observation  of  Professor  Eamsay^'  The 
river  bends;  the  Horseshoe  immediately  accommodates  it- 
self to  the  bending,  and  will  follow  implicitly  the  direction 
of  the  deepest  water  in  the  upper  stream.  The  flexures  of 
the  gorge  are  determined  by  those  of  the  river  channel 
above  it.  Were  the  Niagara  centre  above  the  fall  sinuous, 
the  gorge  would  obediently  follow  its  sinuositieSo      Once 


*  His  words  are:  "Where  the  body  of  water  is  small  in  the  American  Fall, 
the  edge  has  only  receded  a  few  yards  (where  most  eroded)  during  the  time  that 
the  Canadian  Fall  has  receded  from  the  north  corner  of  Goat  Island  to  the 
innermost  curve  of  the  Horseshoe  Fall." — Qum-terly  Journal  of  Geological 
Society,  May,  1869. 


214  FRAGMENTS   OF  SCIENCE 

suggested,  no  doubt  geographers  will  be  able  to  point  out 
many  examples  of  this  action.  The  Zambesi  is  thought 
to  present  a  great  difficulty  to  the  erosion  theory,  because 
of  the  sinaosity  of  the  chasm  below  the  Victoria  Falls. 
Bnt,  assuming  the  basalt  to  be  of  tolerably  uniform  text- 
ure, had  the  river  been  examined  before  the  formation  of 
this  sinuous  channel,  the  present  zigzag  course  of  the  gorge 
below  the  fall  could,  I  am  persuaded,  have  been  predicted, 
while  the  sounding  of  the  present  river  would  enable  us 
to  predict  the  course  to  be  pursued  by  the  erosion  in  the 
future. 

But  not  only  has  the  Niagara  Kiver  cut  the  gorge;  it 
has  carried  away  the  chips  of  its  own  workshop.  The 
shale,  being  probably  crumbled,  is  easily  carried  away. 
But  at  the  base  of  the  fall  we  find  the  huge  bowlders 
already  described,  and  by  some  means  or  other  these  are 
removed  down  the  river.  The  ice  which  fills  the  gorge  in 
winter,  and  which  grapples  with  the  bowlders,  has  been 
regarded  as  the  transporting  agent.  Probably  it  is  so  to 
some  extent.  But  erosion  acts  without  ceasing  on  the 
abutting  points  of  the  bowlders,  thus  withdrawing  their 
support  and  urging  them  gradually  down  the  river.  Solu- 
tion also  does  its  portion  of  the  work.  That  solid  matter 
is  carried  down  is  proved  by  the  difference  of  depth  be- 
tween the  Niagara  River  and  Lake  Ontario,  where  the  river 
enters  it.  The  depth  falls  from  72  feet  to  20  feet,  in  con- 
sequence of  the  deposition  of  solid  matter  caused  by  the 
diminished  motion  of  the  river.' 

The  annexed  highly  instructive  map  has  been  reduced 


*  Near  the  mouth  of  the  gorge  at  Queenston,  the  depth,  according  to  the 
Admiralty  Chart,  is  180  feet;  well  within  the  gorge  it  is  132  feet. 


NIAGARA 


215 


from  one  publislied  in  Mr.  Hall's  ''Geology  of  New  York." 
It  is  based  on  surveys  executed,  in  1842,  by  Messrs.  Gib- 


Fio.  4. 


Bon  and  Eversbed.     Tbe  ragged  edge  of  tbe  American  Fall 
north  of  Goat  Island  marks  the  amount  of  erosion  which 


216  FRAGMENTS   OF  SCIENCE 

it  has  been  able  to  accomplisli,  wbile  the  Horseshoe  Fall 
was  cutting  its  way  southward  across  the  end  of  Goat 
Island  to  its  present  position.  The  American  Fall  is  168 
feet  high,  a  precipice  cut  down,  not  by  itself,  but  by  the 
Horseshoe  Fall.  The  latter,  in  1842,  was  159  feet  high, 
and,  ds  shown  by  the  map,  is  already  turning  eastward,  to 
excavate  its  gorge  along  the  centre  of  the  upper  river, 
p  is  the  apex  of  the  Horseshoe,  and  T  marks  the  site  of 
the  Terrapin  Tower,  with  the  promontory  adjacent,  round 
which  I  was  conducted  by  Conroy.  Probably  since  1842 
the  Horseshoe  has  worked  back  beyond  the  position  here 
assigned  to  it. 

In  conclusion,  we  may  say  a  word  regarding  the  proxi- 
mate future  of  Niagara.  At  the  rate  of  excavation  as- 
^gned  to  it  by  Sir  Charles  Lyell,  namely,  a  foot  a  year, 
five  thousand  years  or  so  will  carry  the  Horseshoe  Fall 
far  higher  than  Goat  Island.  As  the  gorge  recedes  it  will 
drain,  as  it  has  hitherto  done,  Jihe  banks  right  and  left  of 
it,  thus  leaving  a  nearly  level  terrace  between  Goat  Island 
and  the  edge  of  the  gorge.  Higher  up  it  will  totally  drain 
the  American  branch  of  the  river;  the  channel  of  which  in 
due  time  will  become  cultivable  land.  The  American  Fall 
will  then  be  transformed  into  a  dry  precipice,  forming  a 
simple  continuation  of  the  cliffy  boundary  of  the  Niagara 
gorge.  At  the  place  occupied  by  the  fall  at  this  moment 
we  shall  have  the  gorge  enclosing  a  right  angle,  a  second 
whirlpool  being  the  consequence.  To  those  who  visit  Ni- 
agara a  few  millenniums  hence  I  leave  the  verification  of 
this  prediction.  All  that  can  be  said  is,  that  if  the  causes 
now  in  action  continue  to  act,  it  will  prove  itself  literally 
true. 


NIAGARA  217 


Postscript 

A  year  or  so  after  I  had  quitted  the  United  States,  a 
man  sixty  years  of  age,  while  engaged  in  painting  one  of 
the  bridges  which  connect  Groat  Island  with  the  Three  Sis- 
ters, slipped  through  the  rails  of  the  bridge  into  the  rapids, 
and  was  carried  impetuously  toward  the  Horseshoe  Fall. 
He  was  urged  against  a  rock  which  rose  above  the  water, 
and  with  the  grasp  of  desperation  he  clung  to  it.  The 
population  of  the  village  of  Niagara  Falls  was  soon  upon 
the  island,  and  ropes  were  brought,  but  there  was  none  to 
use  them.  In  the  midst  of  the  excitement,  a  tall,  powerful 
young  fellow  was  observed  making  his  way  silently  through 
the  crowd.  He  reached  a  rope;  selected  from  the  bystand- 
ers a  number  of  men,  and  placed  one  end  of  the  rope  in 
their  hands.  The  other  end  he  fastened  round  himself,  and 
choosing  a  point  considerably  above  that  to  which  the  man 
clung,  he  plunged  into  the  rapids.  He  was  carried  vio- 
lently downward,  but  he  caught  the  rock,  secured  the  old 
painter  and  saved  him.  Newspapers  from  all  parts  of  the 
Union  poured  in  upon  me,  describing  this  gallant  act  of 
mj  guide  Conroy. 


SOIENOI^V— 10 


YIII 

THE  PARALLEL  ROADS  OF  GLEN  ROY* 

THE  first  publislied  allusion  to  tlie  Parallel  Eoads  of 
Glen  Koy  occurs  in  the  appendix  to  tlie  third  vol- 
nme  of  Pennant's  "Tour  in  Scotland,"  a  work 
published  in  1776.  "In  the  face  of  these  hills, "  says  this 
writer,  "both  sides  of  the  glen,  there  are  three  roads  at 
small  distances  from  each  other  and  "directly  opposite  on 
each  side.  These  roads  have  been  measured  in  the  com- 
plete parts  of  them,  and  found  to  be  26  paces  of  a  man 
5  feet  10  inches  high.  The  two  highest  are  pretty  near 
each  other,  about  50  yards,  and  the  lowest  double  that 
distance  from  the  nearest  to  it.  They  are  carried  along 
the  sides  of  the  glen  with  the  utmost  regularity,  nearly 
as  exact  as  if  drawn  with  a  line  of  rule  and  compass." 

The  correct  heights  of  the  three  roads  of  Grlen  Roy 
are  respectively  1,150,  1,070,  and  860  feet  above  the  sea. 
Hence  a  vertical  distance  of  80  feet  separates  the  two 
highest,  while  the  lowest  road  is  210  feet  below  the 
middle  one. 

These  "roads"  are  usually  shelves  or  terraces  formed 
in  the  yielding  drift  which  here  covers  the  slopes  of  the 
mountains.  They  are  all  sensibly  horizontal  and  there- 
fore  parallel.      Pennant   accepted    as   reasonable   the    ex- 

'  A  discourse  delivered  at  the  Royal  Institution  of  Great  Britain  on  June  9, 
1876, 

(218) 


THE   PARALLEL    ROADS    OF  GLEN  ROY  219 

planation  of  them  given  by  the  country  people  in  his 
time.  They  thought  that  the  roads  "were  designed  for 
the  chase,  and  that  the  terraces  were  made  after  the  spots 
were  cleared  in  lines  from  wood,  in  order  to  tempt  the 
animals  into  the  open  paths  after  they  were  roused,  in 
order  that  they  might  come  within  reach  of  the  bowmen 
who  might  conceal  themselves  in  the  woods  above  and 
below." 

In  these  attempts  of  "the  country  people"  we  have 
an  illustration  of  that  impulse  to  which  all  scientific 
knowledge  is  due — the  desire  to  know  the  causes  of 
things;  and  it  is  a  matter  of  surprise  that  in  the  case 
of  the  parallel  roads,  with  their  weird  appearance  chal- 
lenging inquiry,  this  impulse  did  not  make  itself  more 
rapidly  and  energetically  felt.  Their  remoteness  may  per- 
haps account  for  the  fact  that  until  the  year  1817  no  sys- 
tematic description  of  them,'  and  no  scientific  attempt  at 
an  explanation  of  them,  appeared.  In  that  year  Dr.  Mac- 
Culloch,  who  was  then  President  of  the  Geological  So- 
ciety, presented  to  that  society  a  memoir,  in  which  the 
roads  were  discussed,  and  pronounced  to  be  the  margins 
of  lakes  once  embosomed  in  Grlen  Roy.  Why  there 
should  be  three  roads,  or  why  the  lakes  should  stand  at 
these  particular  levels,  was  left  unexplained. 

To  Dr.  MacCulloch  succeeded  a  man,  possibly  not  so 
learned  as  a  geologist,  but  obviously  fitted  by  nature 
to  grapple  with  her  facts  and  to  put  them  in  their  proper 
setting.  I  refer  to  Sir  Thomas  Dick-Lauder,  who  pre- 
sented to  the  Royal  Society  of  Edinburgh,  on  the  2d  of 
March,  1818,  his  paper  on  the  Parallel  Roads  of  Glen 
Roy.  In  looking  over  the  literature  of  this  subject,  which 
is  now  copious,  it  is  interesting  to  observe  the  differentia- 


220  FRAGMENTS   OF  SCIENCE 

tion  of  minds,  and  to  single  out  those  who  went  by  a 
kind  of  instinct  to  the  core  of  the  question,  from  those 
who  erred  in  it,  or  who  learnedly  occupied  themselves 
with  its  analogies,  adjuncts,  and  details.  There  is  no 
man,  in  my  opinion,  connected  with  the  history  of  the 
subject,  who  has  shown,  in  relation  to  it,  this  spirit  of 
penetration,  this  force  of  scientific  insight,  more  conspicu- 
ously than  Sir  Thomas  Dick-Lauder.  Two  distinct  men- 
tal processes  are  involved  in  the  treatment  of  such  a  ques- 
tion. First,  the  faithful  and  sufficient  observation  of  the 
data;  and  secondly,  that  higher  mental  process  in  which 
the  constructive  imagination  comes  into  play,  connecting 
the  separate  facts  of  observation  with  their  common  cause, 
and  weaving  them  into  an  organic  whole.  In  neither  of 
these  requirements  did  Sir  Thomas  Dick-Lauder  fail. 

Adjacent  to  Glen  Eoy  is  a  valley  called  Glen  Gluoy, 
along  the  sides  of  which  ran  a  single  shelf,  or  terrace, 
formed  obviously  in  the  same  manner  as  the  parallel  roads 
of  Glen  Koy.  The  two  shelves  on  the  opposing  sides  of 
the  glen  were  at  precisely  the  same  level,  and  Dick-Lauder 
wished  to  see  whether,  and  how,  they  became  united  at 
the  head  of  the  glen.  He  followed  the  shelves  into  the 
recesses  of  the  mountains.  The  bottom  of  the  valley,  as 
it  rose,  came  ever  nearer  to  them,  until  finally,  at  the 
head  of  Glen  Gluoy,  he  reached  a  col,  or  watershed,  of 
precisely  the  same  elevation  as  the  road  which  swept 
round  the  glen. 

The  correct  height  of  this  col  is  1,170  feet  above  the 
sea;  that  is  to  say,  20  feet  above  the  highest  road  in  Glen 
Roy. 

From  this  col  a  lateral  branch -valley — Glen  Turrit — 
led   down   to   Glen   Roy.      Our  explorer  descended  from 


THE  PARALLEL    ROADS   OF   OLEN  ROY  221 

the  col  to  the  highest  road  of  the  latter  glen,  and  pursued 
it  exactly  as  he  had  pursued  the  road  in  Glen  Gluoj. 
For  a  time  it  belted  the  mountain  sides  at  a  considerable 
height  above  the  bottom  of  the  valley;  but  this  rose  as 
he  proceeded,  coming  ever  nearer  to  the  highest  shelf, 
until  finally  he  reached  a  col,  or  water- shed,  looking  into 


PARALLBL  B0AD8  OF  OLBN  ROY. 

After  a  Sketch  by  Sir  Thomas  Dick-Lauder. 

Glen  Spey,    and  of  precisely  the   same  elevation  as  the 
highest  road  of  Glen  Roy. 

He  then  dropped  down  to  the  lowest  of  these  roads, 
and  followed  it  toward  the  mouth  of  the  glen.  Its  eleva- 
tion above  the  bottom  of  the  valley  gradually  increased; 
not  because  the  shelf  rose,  but  because  it  remained  level 
while  the  valley  sloped  downward.  He  found  this  low- 
est road  doubling  round  the  hills  at  the  mouth  of  Glen 
Roy,  and  running  along  the  sides  of  the  mountains  which 


222  FRAGMENTS   OF  SCIENCE 

flank  Glen  Spean.  He  followed  it  eastward.  The  bottom 
of  the  Spean  Valley,  like  the  others,  gradually  rose,  and 
therefore  gradually  approached  the  road  on  the  adjacent 
mountain- side.  He  came  to  Loch  Laggan,  the  surface  of 
which  rose  almost  to  the  level  of  the  road,  and  beyond 
the  head  of  this  lake  he  found,  as  in  the  other  two  cases, 
a  col,  or  watershed,  at  Makul,  of  exactly  the  same  level 
as  the  single  road  in  Glen  Spean,  which,  it  will  be  remem- 
bered, is  a  continuation  of  the  lowest  road  in  Glen  Roy. 

Here  we  have  a  series  of  facts  of  obvious  significance 
as  regards  the  solution  of  this  problem.  The  effort  of  the 
mind  to  form  a  coherent  image  from  such  facts  may  be 
compared  with  the  effort  of  the  eyes  to  cause  the  pictures 
of  a  stereoscope  to  coalesce.  For  a  time  we  exercise 
a  certain  strain,  the  object  remaining  vague  and  indis- 
tinct. Suddenly  its  various  parts  seem  to  run  together, 
the  object  starting  forth  in  clear  and  definite  relief. 
Such,  I  take  it,  was  the  effect  of  his  ponderings  upon 
the  mind  of  Sir  Thomas  Dick -Lauder.  His  solution  was 
this:  Taking  all  their  features  into  account,  he  was  con- 
vinced that  water  only  could  have  produced  the  terraces. 
But  how  had  the  water  been  collected?  He  saw  clearly 
that,  supposing  the  mouth  of  Glen  Gluoy  to  be  stopped 
by  a  barrier  sufficiently  high,  if  the  waters  from  the 
mountains  flanking  the  glen  were  allowed  to  collect,  they 
would  form  behind  the  barrier  a  lake,  the  surface  of  which 
would  gradually  rise  until  it  reached  the  level  of  the  col 
at  the  head  of  the  glen.  The  rising  would  then  cease ;  the 
superfluous  water  of  Glen  Gluoy  discharging  itself  over 
the  «ol  into  Glen  Roy.  As  long  as  the  barrier  stopping 
the  mouth  of  Glen  Gluoy  continued  high  enough,  we 
should  have  in   that  glen  a  lake   at  the  precise  level  of 


THE  PARALLEL   ROADS   OF  QLEN  ROY  223 

its  shelf,  which  lake,  acting  upon  the  loose  drift  of  the 
flanking  mountains,  would  form  the  shelf  revealed  by 
observation. 

So  much  for  Glen  Gluoy.  But  suppose  the  mouth  of 
Glen  Eoy  also  stopped  by  a  similar  barrier.  Behind  it 
also  the  water  from  the  adjacent  mountains  would  collect. 
The  surface  of  the  lake  thus  formed  would  gradually  rise, 
until  it  had  reached  the  level  of  the  col  which  divides 
Glen  Eoy  from  Glen  Spey.  Here  the  rising  of  the  lake 
would  cease;  its  superabundant  water  being  poured  over 
the  col  into  the  valley  of  the  Spey.  This  state  of  things 
would  continue  as  long  as  a  sufficiently  high  barrier  re- 
mained at  the  mouth  of  Glen  Roy.  The  lake  thus  dammed 
in,  with  its  surface  at  the  level  of  the  highest  parallel 
road,  would  act,  as  in  Glen  Gluoy,  upon  the  friable  drift 
overspreading  the  mountains,  and  would  form  the  highest 
road  or  terrace  of  Glen  Roy. 

And  now  let  us  suppose  the  barrier  to  be  so  far  re- 
moved from  the  mouth  of  Glen  Roy  as  to  establish  a  con- 
nection between  it  and  the  upper  part  of  Glen  Spean, 
while  the  lower  part  of ,  the  latter  glen  still  continued  to 
be  blocked  up.  Upper  Glen  Spean  and  Glen  Roy  would 
then  be  occupied  by  a  continuous  lake,  the  level  of  which 
would  obviously  be  determined  by  the  col  at  the  head  of 
Loch  Laggan.  The  water  in  Glen  Roy  would  sink  from 
the  level  it  had  previously  maintained  to  the  level  of 
its  new  place  of  escape.  This  new  lake-surface  would 
correspond  exactly  with  the  lowest  parallel  road,  and  it 
would  form  that  road  by  its  action  upon  the  drift  of  the 
adjacent  mountains. 

In  presence  of  the  observed  facts,  this  solution  com- 
mends itself  strongly  to  the  scientific  mind.      The  ques- 


224  FRAGMENTS   OF  SCIENCE 

tion  next  occurs,  What  was  the  character  of  the  assumed 
barrier  which  stopped  the  glens?  There  are  at  the  pres- 
ent moment  vast  masses  of  detritus  in  certain  portions  of 
Glen  Spean,  and  of  such  detritus  Sir  Thomas  Dick-Lauder 
imagined  his  barriers  to  have  been  formed.  By  some  un- 
known convulsion,  this  detritus  had  been  heaped  up. 
But,  once  given,  and  once  granted  that  it  was  subse- 
quently removed  in  the  manner  indicated,  the  single  road 
of  Glen  Gluoy  and  the  highest  and  lowest  roads  of  Glen 
Eoy  would  be  explained  in  a  satisfactory  manner. 

To  account  for  the  second  or  middle  road  of  Glen  Roy, 
Sir  Thomas  Dick-Lauder  invoked  a  new  agency.  He  sup- 
posed that  at  a  certain  point  in  the  breaking  down  or 
waste  of  his  dam,  a  halt  occurred,  the  barrier  holding  its 
ground  at  a  particular  level  sufficiently  long  to  dam  a 
lake  rising  to  the  height  of,  and  forming  the  second  road. 
This  point  of  weakness  was  at  once  detected  by  Mr.  Dar- 
win, and  adduced  by  him  as  proving  that  the  levels  of 
the  cols  did  not  constitute  an  essential  feature  in  the 
phenomena  of  the  parallel  roads.  Though  not  destroyed, 
Sir  Thomas  Dick-Lauder's  theory  was  seriously  shaken 
by  this  argument,  and  it  became  a  point  of  capital  im- 
portance, if  the  facts  permitted,  to  remove  such  source 
of  weakness.  This  was  done  in  1847  by  Mr.  David  Milne, 
now  Mr.  Milne- Home.  On  walking  up  Glen  Roy  from 
Roy  Bridge,  we  pass  the  mouth  of  a  lateral  glen,  called 
Glen  Glaster,  running  eastward  from  Glen  Roy.  There 
is  nothing  in  this  lateral  glen  to  attract  attention,  or  to 
suggest  that  it  could  have  any  conspicuous  influence  in 
the  production  of  the  parallel  roads.  Hence,  probably, 
the  failure  of  Sir  Thomas  Dick-Lauder  to  notice  it.  But 
Mr.  Milne- Home  entered  this  glen,  on  the  northern  side  ol 


THE  PARALLEL   ROADS   OF  GLEN  ROY  225 

which  the  middle  and  lowest  roads  are  fairly  shown.  The 
principal  stream  running  through  the  glen  turns  at  a  cer- 
tain point  northward  and  loses  itself  among  hills  too  high 
to  offer  any  outlet.  But  another  branch  of  the  glen  turns 
to  the  southeast;  and,  following  up  this  branch,  Mr. 
Milne-Home  reached  a  col,  or  watershed,  of  the  precise 
level  of  the  second  Glen  Eoy  road.  When  the  barrier 
blocking  the  glens  had  been  so  far  removed  as  to  open 
this  col,  the  water  in  Glen  Eoy  would  sink  to  the  level 
of  the  second  road.  A  new  lake  of  diminished  depth 
would  be  thus  formed,  the  surplus  water  of  which  would 
escape  over  the  Glen  Glaster  col  into  Glen  Spean.  The 
margin  of  this  new  lake,  acting  upon  the  detrital  matter, 
would  form  the  second  road.  The  theory  of  Sir  Thomas 
Dick-Lauder,  as  regards  the  part  played  by  the  cols,  was 
re-riveted  by  this  new  and  unexpected  discovery. 

I  have  referred  to  Mr.  Darwin,  whose  powerful  mind 
swayed  for  a  time  the  convictions  of  the  scientific  world 
in  relation  to  this  question.  His  notion  was — and  it  is  a 
notion  which  very  naturally  presents  itself — ^that  the  par- 
allel roads  were  formed  by  the  sea;  that  this  whole  region 
was  once  submerged  and  subsequently  upheaved;  that 
there  were  pauses  in  the  process  of  upheaval,  during 
which  these  glens  constituted  so  many  fiords,  on  the  sides 
of  which  the  parallel  terraces  were  formed.  This  theory 
will  not  bear  close  criticism;  nor  is  it  now  maintained  by 
Mr.  Darwin  himself.  It  would  not  account  for  the  sea 
being  20  feet  higher  in  Glen  Gluoy  than  in  Glen  Eoy. 
It  would  not  account  for  the  absence  of  the  second  and 
third  Glen  Eoy  roads  from  Glen  Gluoy,  where  the  moun- 
tain flanks  are  quite  as  impressionable  as  in  Glen  Eoy. 
It  would  not  account  for  the  absence  of  the  shelves  from 


226  FRAGMENTS    OF  SCIENCE 

the  other  mountains  in  the  neighborhood,  all  of  which 
would  have  been  clasped  by  the  sea  had  the  sea  been 
there.  Here  then,  and  no  doubt  elsewhere,  Mr.  Darwin 
has  shown  himself  to  be  fallible;  but  here,  as  elsewhere, 
he  has  shown  himself  equal  to  that  discipline  of  surrender 
to  evidence  which  girds  his  intellect  with  such  unassail- 
able moral  strength. 

But,  granting  the  significance  of  Sir  Thomas  Dick- 
Lauder's  facts,  and  the  reasonableness,  on  the  whole,  of 
the  views  which  he  has  founded  on  them,  they  will  not 
bear  examination  in  detail.  No  such  barriers  of  detritus 
as  he  assumed  could  have  existed  without  leaving  traces 
behind  them ;  but  there  is  no  trace  left.  There  is  detritus 
enough  in  Glen  Spean,  but  not  where  it  is  wanted.  The 
two  highest  parallel  roads  stop  abruptly  at  different  points 
near  the  mouth  of  Glen  Roy,  but  no  remnant  of  the  bar- 
rier against  which  they  abutted  is  to  be  seen.  It  might 
be  urged  that  the  subsequent  invasion  of  the  valley  by 
glaciers  has  swept  the  detritus  away;  but  there  have  been 
no  glaciers  in  these  valleys  since  the  disappearance  of  the 
lakes.  Professor  Geikie  has  favored  me  with  a  drawing 
of  the  Glen  Spean  '''road"  near  the  entrance  to  Glen 
Trieg.  The  road  forms  a  shelf  round  a  great  mound  of 
detritus  which,  had  a  glacier  followed  the  formation  of  the 
shelf,  must  have  been  cleared  away.  Taking  all  the  cir- 
cumstances into  account,  you  may,  I  think,  with  safety 
dismiss  the  detrital  barrier  as  incompetent  to  account  for 
the  present  condition  of  Glen  Gluoy  and  Glen  Roy. 

Hypotheses  in  science,  though  apparently  transcending 
experience,  are  in  reality  experience  modified  by  scien- 
tific thought  and  pushed  into  an  ultra  experiential  region. 
At  the  time  that  he  wrote.  Sir  Thomas  Dick-Lauder  could 


THE  PARALLEL   ROADS   OF   GLEN  ROY  227 

not  possibly  have  discerned  the  cause  subsequently  as- 
signed for  the  blockage  of  these  glens.  A  knowledge  of 
the  action  of  ancient  glaciers  was  the  necessary  ante- 
cedent to  the  new  explanation,  and  experience  of  this 
nature  was  not  possessed  by  the  distinguished  writer  just 
mentioned.  The  extension  of  Swiss  glaciers  far  beyond 
their  present  limits  was  first  made  known  by  a  Swiss 
engineer  named  Yenetz,  who  established,  by  the  marks 
they  had  left  behind  them,  their  former  existence  in  places 
which  they  had  long  forsaken.  The  subject  of  glacier 
extension  was  siibsequently  followed  up  with  distin- 
guished success  by  Charpentier,  Studer,  and  others.  With 
characteristic  vigor  Agassiz  grappled  with  it,  extending 
his  observations  far  beyond  the  domain  of  Switzerland. 
He  came  to  this  country  in  1840,  and  found  in  various 
places  indubitable  marks  of  ancient  glacier  action.  Eng- 
land, Scotland,  Wales,  and  Ireland  he  proved  to  have 
once  given  birth  to  glaciers.  He  visited  Glen  Roy,  sur- 
veyed the  surrounding  neighborhood,  and  pronounced,  as 
a  consequence  of  his  investigation,  the  barriers  which 
stopped  the  glens  and  produced  the  parallel  roads  to 
have  been  barriers  of  ice.  To  Mr.  Jamieson,  above  all 
others,  we  are  indebted  for  the  thorough  testing  and  con- 
firmation of  this  theory. 

And  let  me  here  say  that  Agassiz  is  only  too  likely  to 
be  misrated  and  misjudged  ty  those  who,  though  accurate 
within  a  limited  sphere,  fail  to  grasp  in  their  totality  the 
motive  powers  invoked  in  scientific  investigation.  True, 
he  lacked  mechanical  precision,  but  he  abounded  in  that 
force  and  freshness  of  the  scientific  imagination  which  in 
some  sciences,  and  probably  in  some  stages  of  all  sciences, 
are  essential  to  the  creator  of  knowledge.     To  Agassiz  was 


228  FRAGMENTS    OF  SCIENCE 

given,  not  the  art  of  the  refiner,  but  the  instinct  of  the 
discoverer,  and  the  strength  of  the  delver  who  brings  ore 
from  the  recesses  of  the  mine.  That  ore  may  contain  its 
share  of  dross,  but  it  also  contains  the  precious  metal 
which  gives  employment  to  the  refiner,  and  without  which 
his  occupation  would  depart. 

Let  us  dwell  for  a  moment  upon  this  subject  of  ancient 
glaciers.  Under  a  flask  containing  water,  in  which  a  ther- 
mometer is  immersed,  is  placed  a  Bunsen's  lamp.  The  wa- 
ter is  heated,  reaches  a  temperature  of  212°,  and  then  begins 
to  boil.  The  rise  of  the  thermometer  then  ceases,  although 
heat  continues  to  be  poured  by  the  lamp  into  the  water. 
What  becomes  of  that  heat?  We  know  that  it  is  con- 
sumed in  the  molecular  work  of  vaporization.  In  the  ex- 
periment here  arranged,  the  steam  passes  from  the  flask 
through  a  tube  into  a  second  vessel  kept  at  a  low  tem- 
perature. Here  it  is  condensed,  and  indeed  congealed  to 
ice,  the  second  vessel  being  plunged  in  a  mixture  cold 
enough  to  freeze  the  water.  As  a  result  of  the  process 
we  obtain  a  mass  of  ice.  That  ice  has  an  origin  very 
antithetical  to  its  own  character.  Though  cold,  it  is  the 
child  of  heat.  If  we  removed  the  lamp,  there  would  be 
no  steam,  and  if  there  were  no  steam  there  would  be  no 
ice.  The  mere  cold  of  the  mixture  surrounding  the  sec- 
ond vessel  would  not  produce  ice.  The  cold  must  have 
the  proper  material  to  work  upon;  and  this  material — 
aqueous  vapor — is,  as  we  here  see,  the  direct  product 
of  heat. 

It  is  now,  I  suppose,  fifteen  or  sixteen  years  since  I 
found  myself  conversing  with  an  illustrious  philosopher 
regarding  that  glacial  epoch  which  the  researches  of  Agas- 
siz  and  others  have  revealed.     This  profoundly  thoughtful 


THE  PARALLEL   ROADS   OF  GLEN  ROY  229 

man  maintained  the  fixed  opinion  that,  at  a  certain  stage 
in  the  history  of  the  solar  system,  the  sun's  radiation  had 
Buffered  diminution,  the  glacial  epoch  being  a  consequence 
of  this  solar  chill.  The  celebrated  French  mathematician, 
Poisson,  had  another  theory.  Astronomers  have  shown 
that  the  solar  system  moves  through  space,  and  '*the  tem- 
perature of  space"  is  a  familiar  expression  with  scientific 
men.  It  was  considered  probable  by  Poisson  that  our  sys- 
tem, during  its  motion,  had  traversed  portions  of  space 
of  different  temperatures;  and  that,  during  its  passage 
through  one  of  the  colder  regions  of  the  universe,  the 
glacial  epoch  occurred.  Notions  such  as  these  were  more 
or  less  current  everywhere  not  many  years  ago,  and  I 
therefore  thought  it  worth  while  to  show  how  incomplete 
they  were.  Suppose  the  temperature  of  our  planet  to  be 
reduced,  by  the  subsidence  of  solar  heat,  the  cold  of  space, 
or  any  other  cause,  say  one  hundred  degrees.  Four-and- 
twenty  hours  of  such  a  chill  would  bring  down  as  snow 
nearly  all  the  moisture  of  our  atmosphere.  But  this 
would  not  produce  a  glacial  epoch.  Such  an  epoch  would 
require  the  long-continued  generation  of  the  material  from 
which  the  ice  of  glaciers  is  derived.  Mountain  snow,  the 
nutriment  of  glaciers,  is  derived  from  aqueous  vapor  raised 
mainly  from  the  tropical  ocean  by  the  sun.  The  solar  fire 
is  as  necessary  a  factor  in  the  process  as  our  lamp  in  the 
experiment  referred  to  a  moment  ago.  Nothing  is  easier 
than  to  calculate  the  exact  amount  of  heat  expended  by 
the  sun  in  the  production  of  a  glacier.  It  would,  as  I 
have  elsewhere  shown,  ^  raise  a  quantity  of  cast  iron  five 


>  "Heat  a  Mode  of  Motion,"  fifth  edition,  chap,  vl.:  Forma  of  Water,  %%  66 
and  66. 


230  FRAGMENTS   OF  SCIENCE 

times  the  weight  of  the  glacier  not  only  to  a  white  heat, 
but  to  its  point  of  fusion.  If,  as  I  have  already  urged, 
instead  of  being  filled  with  ice,  the  valleys  of  the  Alps 
were  filled  with  white-hot  metal,  of  quintuple  the  mass  of 
the  present  glaciers,  it  is  the  heat,  and  not  the  cold,  that 
would  arrest  our  attention  and  solicit  our  explanation. 
The  process  of  glacier  making  is  obviously  one  of  distil- 
lation, in  which  the  fire  of  the  sun,  which  generates  the 
vapor,  plays  as  essential  a  part  as  the  cold  of  the  moun- 
tains which  condenses  it.* 

It  was  their  ascription  to  glacier  action  that  first  gave 
the  parallel  roads  of  Glen  Koy  an  interest  in  my  eyes ;  and 
in  1867,  with  a  view  to  self -instruction,  I  made  a  solitary 
pilgrimage  to  the  place,  and  explored  pretty  thoroughly 
the  roads  of  the  principal  glen.  I  traced  the  highest  road 
to  the  col  dividing  Glen  Eoy  from  Glen  Spey,  and,  thanks 
to  the  civility  of  an  Ordnance  surveyor,  I  was  enabled  to 
inspect  some  of  the  roads  with  a  theodolite,  and  to  satisfy 
myself  regarding  the  common  level  of  the  shelves  at  oppo- 
site sides  of  the  valley.  As  stated  by  Pennant,  the  width 
of  the  roads  amounts  sometimes  to  more  than  twenty  yards ; 
but  near  the  head  of  Glen  Koy  the  highest  road  ceases  to 
have  any  width,  for  it  runs  along  the  face  of  a  rock,  the 
effect  of  the  lapping  of  the  water  on  the  more  friable  por- 
tions of  the  rock  being  perfectly  distinct  to  this  hour. 
My  knowledge  of  the  region  was,  however,  far  from  com' 
plete,  and  nine  years  had  dimmed  the  memory  even  of  the 


^  In  Lyell's  excellent  "Principles  of  Geology,"  the  remark  occurs  that 
•'several  writers  have  fallen  into  the  strange  error  of  supposing  that  the  glacial 
period  must  have  been  one  of  higher  mean  temperature  than  usual."  The 
really  strange  error  was  the  forgetfulness  of  the  fact  that  without  the  heat  the 
aabstance  necessary  to  the  production  of  glaciers  would  be  wanting. 


THE  PARALLEL    ROADS   OF   GLEN   ROY  231 

portion  which  had  been  thoroughly  examined.  Hence  my 
desire  to  see  the  roads  once  more  before  venturing  to  talk 
to  you  about  them.  The  Easter  holidays  of  1876  were  to 
be  devoted  to  this  purpose;  but  at  the  last  moment  a  tele- 
gram from  Roy  Bridge  informed  me  that  the  roads  were 
snowed  up.  Finding  books  and  memories  poor  substitutes 
for  the  flavor  of  facts,  I  resolved  subsequently  to  make 
another  effort  to  see  the  roads.  Accordingly  last  Thurs- 
day fortnight,  after  lecturing  here,  I  packed  up,  and  started 
(not  this  time  alone)  for  the  North.  Next  day  at  noon 
my  wife  and  I  found  ourselves  at  Dalwhinnie,  whence  a 
drive  of  some  five- and -thirty  miles  brought  us  to  the  ex- 
cellent hostlery  of  Mr.  Macintosh,  at  the  mouth  of  Glen 
Roy. 

We  might  have  found  the  hills  covered  with  mist, 
which  would,  have  wholly  defeated  us;  but  Nature  was 
good-natured,  and  we  had  two  successful  working  days 
among  the  hills.  Guided  by  the  excellent  ordnance  map 
of  the  region,  on  the  Saturday  morning  we  went  up  the 
glen,  and  on  reaching  the  stream  called  Allt  Bhreac 
Achaidh  faced  the  hills  to  the  west.  At  the  watershed 
between  Glen  Roy  and  Glen  Fintaig  we  bore  northward, 
struck  the  ridge  above  Glen  Gluoy,  and  came  in  view  of 
its  road,  which  we  persistently  followed  as  long  as  it  con- 
tinued visible.  It  is  a  feature  of  all  the  roads  that  they 
vanish  before  reaching  the  cols  over  which  fell  the  waters 
of  the  lakes  which  formed  them.  One  reason  doubtless 
is  that  at  their  upper  ends  the  lakes  were  shallow,  and 
incompetent  on  this  account  to  raise  wavelets  of  any 
strength  to  act  upon  the  mountain  drift.  A  second  rea- 
son is  that  they  were  land-locked  in  the  higher  portions 
and  protected  from  the  southwesterly  winds,  the  stillness 


282  FRAGMENTS   OF  SCIENCE 

of  their  waters  causing  tliem  to  produce  but  a  feeble  im- 
pression upon  the  mountain  sides.  From  Glen  Gluoy  we 
passed  down  Glen  Turrit  to  Glen  Roy,  and  through  it 
homeward,  thus  accomplishing  two  or  three  and  twenty 
miles  of  rough  and  honest  work. 

Next  day  we  thoroughly  explored  Glen  Glaster,  follow- 
ing its  two  roads  as  far  as  they  were  visible.  We  reached 
the  col  discovered  by  Mr.  Milne-Home,  which  stands  at 
the  level  of  the  middle  road  of  Glen  Roy.  Thence  we 
crossed  southward  over  the  mountain  Creag  Dhuhh^  and 
examined  the  erratic  blocks  upon  its  sides,  and  the  ridges 
and  mounds  of  moraine  matter  which  cumber  the  lower 
flanks  of  the  mountain.  The  observations  of  Mr.  Jamie - 
son  upon  this  region,  including  the  mouth  of  Glen  Trieg, 
are  in  the  highest  degree  interesting.  We  entered  Glen 
Spean,  and  continued  a  search  begun  on  the  evening  of 
our  arrival  at  Roy  Bridge — the. search,  namely,  for  glacier 
polishings  and  markings.  We  did  not  find  them  copious, 
but  they  are  indubitable.  One  of  the  proofs  most  con- 
venient for  reference  is  a  great  rounded  rock  by  the 
roadside,  1,000  yards  east  of  the  milestone  marked  three- 
quarters  of  a  mile  from  Roy  Bridge.  Further  east  other 
cases  occur,  and  they  leave  no  doubt  upon  the  mind  that 
Glen  Spean  was  at  one  time  filled  by  a  great  glacier.  To 
the  disciplined  eye  the  aspect  of  the  mountains  is  perfectly 
conclusive  on  this  point;  and  in  no  position  can  the  ob- 
server more  readily  and  thoroughly  convince  himself  of 
this  than  at  the  head  of  Glen  Glaster.  The  dominant 
hills  here  are  all  intensely  glaciated. 

But  the  great  collecting  ground  of  the  glaciers  which 
dammed  the  glens  and  produced  the  parallel  roads  were 
the  mountains  south  and  west  of  Glen  Spean.     The  mon- 


THE   PARALLEL    ROADS    OF   GLEN  ROY  233 

arch  of  tliese  is  Ben  Nevis,  4,370  feet  high.  The  position 
of  Ben  Nevis  and  his  colleagues,  in  reference  to  the  vapor- 
laden  winds  of  the  Atlantic,  is  a  point  of  the  first  impor- 
tance. It  is  exactly  similar  to  that  of  Carrantnal  and  the 
Macgillicnddy  Eeeks  in  the  southwest  of  Ireland.  These 
mountains  are,  and  were,  the  first  to  encounter  the  south- 
western Atlantic  winds,  and  the  precipitation,  even  at 
present,  in  the  neighborhood  of  Killarney,  is  enormous. 
The  winds,  robbed  of  their  vapor,  and  charged  with  the 
heat  set  free  by  its  precipitation,  pursue  their  direction 
obliquely  across  Ireland;  and  the  effect  of  the  drying 
process  may  be  understood  by  comparing  the  rainfall  at 
Cahirciveen  with  that  at  Portarlington.  As  found  ^  by  Dr. 
Lloyd,  the  ratio  is  as  59  to  21 — fifty-nine  inches  annually 
at  Cahirciveen  to  twenty -one  at  Portarlington.  During  the 
glacial  epoch  this  vapor  fell  as  snow,  and  the  consequence 
was  a  system  of  glaciers  which  have  left  traces  and  evi- 
dences of  the  most  impressive  character  in  the  region  of 
the  Killarney  Lakes.  I  have  referred  in  other  places  to 
the  great  glacier  which,  descending  from  the  Keeks,  moved 
through  the  Black  Valley,  took  possession  of  the  lake- 
basins,  and  left  its  traces  on  every  rock  and  island  emer- 
gent from  the  waters  of  the  upper  lake.  They  are  all 
conspicuously  glaciated.  Not  in  Switzerland  itself  do  we 
find  clearer  traces  of  ancient  glacier  action. 

What  the  Macgillicuddy  Reeks  did  in  Ireland,  Ben 
Nevis  and  the  adjacent  mountains  did,  and  continue  to 
do,  in  Scotland.  We  had  an  example  of  this  on  the 
morning  we  quitted  Roy  Bridge.  From  the  bridge  west- 
ward rain  fell  copiously,  and  the  roads  were  wet;  but  the 
precipitation  ceased  near  Loch  Laggan,  whence  eastward 
the  roads  were  dry.     Measured  by  the  gauge,  the  rainfall 


284  FRAGMENTS   OF  SCIENCE 

at  Fort  William  is  86  inches,  while  at  Laggan  it  is  only 
46  inches,  annually.  The  difference  between  west  and  east 
is  forcibly  brought  out  by  observations  at  the  two  ends  of 
the  Caledonian  Canal.  Fort  William  at  the  southwestern 
end  has,  as  just  stated,  86  inches,  while  Culloden,  at  its 
northeastern  end,  has  only  24.  To  the  researches  of  that 
able  and  accomplished  meteorologist,  Mr.  Buchan,  we  are 
indebted  for  these  and  other  data  of  the  most  interesting 
and  valuable  kind. 

Adhering  to  the  facts  now  presented  to  us,  it  is  not 
difficult  to  restore  in  idea  the  process  by  which  the  gla- 
ciers of  Lochaber  were  produced  and  the  glens  dammed 
by  ice.  When  the  cold  of  the  glacial  epoch  began  to  in- 
vade the  Scottish  hills,  the  sun  at  the  same  time  acting 
with  sufficient  power  upon  the  tropical  ocean,  the  vapors 
raised  and  drifted  on  to  these  northern  mountains  were 
more  and  more  converted  into  snow.  This  slid  down  the 
slopes,  and  from  every  valley,  strath,  and  corry,  south  of 
Glen  Spean,  glaciers  were  poured  into  that  glen.  The  two 
great  factors  here  brought  into  play  are  the  nutrition  of  the 
glaciers  by  the  frozen  material  above,  and  their  consump- 
tion in  the  milder  air  below.  For  a  period  supply  ex- 
ceeded consumption,  and  the  ice  extended,  filling  Glen 
Spean  to  an  ever-increasing  height,  and  abutting  against 
the  mountains  to  the  north  of  that  glen.  But  why,  it  may 
be  asked,  should  the  valleys  south  of  Glen  Spean  be  re- 
ceptacles of  ice  at  a  time  when  those  north  of  it  were 
receptacles  of  water?  The  answer  is  to  be  found  in  the 
position  and  the  greater  elevation  of  the  mountains  south 
of  Glen  Spean.  They  first  received  the  loads  of  moisture 
carried  by  the  Atlantic  winds,  and  not  until  they  had  been 
in  part  dried,  and  also  warmed  by  the  liberation  of  their 


THE   PARALLEL    ROADS    OF   GLEN  ROY  235 

latent  heat,  did  these  winds  touch  the  hills  north  of  the 
glen. 

An  instructive  observation  bearing  upon  this  point  is 
here  to  be  noted.  Had  our  visit  been  in  the  winter  we 
should  have  found  all  the  mountains  covered:  had  it  been 
in  the  summer  we  should  have  found  the  snow  all  gone. 
But  happily  it  was  at  a  season  when  the  aspect  of  the 
mountains  north  and  south  of  Glen  Spean  exhibited  their 
relative  powers  as  snow  collectors.  Scanning  the  former 
hills  from  many  points  of  view,  we  were  hardly  able  to 
detect  a  fleck  of  snow,  while  heavy  swaths  and  patches 
loaded  the  latter.  Were  the  glacial  epoch  to  return,  the 
relation  indicated  by  this  observation  would  cause  Glen 
Spean  to  be  filled  with  glaciers  from  the  south,  while  the 
hills  and  valleys  on  the  north,  visited  by  warmer  and  drier 
winds,  would  remain  comparatively  free  from  ice.  This 
flow  from  the  south  would  be  reinforced  from  the  west, 
and  as  long  as  the  supply  was  in  excess  of  the  consump- 
tion the  glaciers  would  extend,  the  dams  which  closed  the 
glens  increasing  in  height.  By  and  by  supply  and  con- 
sumption becoming  approximately  equal,  the  height  of  the 
glacier  barriers  would  remain  constant.  Then,  if  milder 
weather  set  in,  consumption  would  be  in  excess,  a  lower- 
ing of  the  barriers  and  a  retreat  of  the  ice  being  the  conse- 
quence. But  for  a  long  time  the  conflict  between  supply 
and  consumption  would  continue,  retarding  indefinitely  the 
disappearance  of  the  barriers,  and  keeping  the  imprisoned 
lakes  in  the  northern  glens.  But  however  slow  its  retreat, 
the  ice  in  the  long  run  would  be  forced  to  yield.  The 
dam  at  the  mouth  of  Glen  Roy,  which  probably  entered 
the  glen  sufficiently  far  to  block  up  Glen  Glaster,  would 
gradually  retreat.     Glen  Glaster  and  its  col  being  opened, 


236  FRAGMENTS   OF  SCIENCE 

the  subsidence  of  the  lake  eighty  feet,  from  the  level  of 
the  highest  to  that  of  the  second  parallel  road,  would  fol- 
low as  a  consequence.  I  think  this  the  most  probable 
course  of  things,  but  it  is  also  possible  that  Glen  Glaster 
may  have  been  blocked  by  a  glacier  from  Glen  Trieg. 
The  ice  dam  continuing  to  retreat,  at  length  permitted 
Glen  Eoy  to  connect  itself  with  upper  Glen  Spean.  A 
continuous  lake  then  filled  both  glens,  the  level  of  which, 
as  already  explained,  was  determined  by  the  col  at  Makul, 
above  the  head  of  Loch  Laggan.  The  last  to  yield  was  the 
portion  of  the  glacier  which  derived  nutrition  from  Ben 
Nevis,  and  probably  also  from  the  mountains  north  and 
south  of  Loch  Arkaig.  But  it  at  length  yielded,  and  the 
waters  in  the  glens  resumed  the  courses  which  they  pursue 
to-day. 

For  the  removal  of  the  ice  barriers  no  cataclysm  is  to 
be  invoked ;  the  gradual  melting  of  the  dam  would  produce 
the  entire  series  of  phenomena.  In  sinking  from  col  to 
col  the  water  would  flow  over  a  gradually  melting  barrier, 
the  surface  of  the  imprisoned  lake  not  remaining  sufficiently 
long  at  any  particular  level  to  produce  a  shelf  comparable 
to  the  parallel  roads.  By  temporary  halts  in  the  process 
of  melting  due  to  atmospheric  conditions  or  to  the  charac- 
ter of  the  dam  itself,  or  through  local  softness  in  the  drift, 
small  pseudo -terraces  would  be  formed,  which,  to  the  per- 
plexity of  some  observers,  are  seen  upon  the  flanks  of  the 
glens  to-day. 

In  presence  then  of  the  fact  that  the  barriers  which 
stopped  these  glens  to  a  height,  it  may  be,  of  1,600  feet 
above  the  bottom  of  Glen  Spean,  have  dissolved  and  left 
not  a  wreck  behind;  in  presence  of  the  fact,  insisted  on 
by  Professor  Geikie,  that  barriers  of  detritus  would  nn- 


THE  PARALLEL   ROADS   OF  GLEN  ROY  237 

doubtedly  have  been  able  to  maintain  themselves  had 
they  ever  been  there;  in  presence  of  the  fact  that  great 
glaciers  once  most  certainly  filled  these  valleys — that  the 
whole  region,  as  proved  by  Mr.  Jamieson,  is  filled  with 
the  traces  of  their  action;  the  theory  which  ascribes 
the  parallel  roads  to  lakes  dammed  by  barriers  of  ice 
has,  in  my  opinion,  a  degree  of  probability  on  its 
side  which  amounts  to  a  practical  demonstration  of  its 
truth. 

Into  the  details  of  the  terrace  formation  I  do  not  enter. 
Mr.  Darwin  and  Mr.  Jamieson  on  the  one  side,  and  Sir 
John  Lubbock  on  the  other,  deal  with  true  causes.  The 
terraces,  no  doubt,  are  due  in  part  to  the  descending  drift 
arrested  by  the  water,  and  in  part  to  the  fretting  of  the 
wavelets,  and  the  rearrangement  of  the  stirred  detritus, 
along  the  belts  of  contact  of  lake  and  hill.  The  descent 
of  matter  must  have  been  frequent  when  the  drift  was 
unbound  by  the  rootlets  which  hold  it  together  now.  In 
some  cases,  it  may  be  remarked,  the  visibility  of  the  roads 
is  materially  augmented  by  differences  of  vegetation.  The 
grass  upon  the  terraces  is  not  always  of  the  same  char- 
acter as  that  above  and  below  them,  while  on  heather- 
covered  hills  the  absence  of  the  dark  shrub  from  the  roads 
greatly  enhances  their  conspicuousness. 

The  annexed  sketch  of  a  model  (p.  238)  will  enable  the 
reader  to  grasp  the  essential  features  of  the  problem  and 
its  solution.  Glen  Gluoy  and  Glen  Roy  are  lateral  val- 
leys which  open  into  Glen  Spean.  Let  us  suppose  Glen 
Spean  filled  from  v  to  w  with  ice  of  a  uniform  elevation 
of  1,500  feet  above  the  sea,  the  ice  not  filling  the  upper 
part  of  that  glen.  The  ice  would  thrust  itself  for  some 
distance  up  the  lateral  valleys,   closing   all  their  mouths. 


240  FRAGMENTS   OF  SCIENCE 

Spean,  the  waters  would  flow  down  their  respective  val- 
leys as  they  do  to-day. 

Reviewing  our  work,  we  find  three  considerable  steps 
to  have  marked  the  solution  of  the  problem  of  the  Par- 
allel Roads  of  Glen  Roy.  The  first  of  these  was  taken 
by  Sir  Thomas  Dick- Lauder,  the  second  was  the  pregnant 
conception  of  Agassiz  regarding  glacier  action,  and  the 
third  was  the  testing  and  verification  of  this  conception 
by  the  very  thorough  researches  of  Mr.  Jamieson.  No 
circumstance  or  incident  connected  with  this  discourse 
gives  me  greater  pleasure  than  the  recognition  of  the 
value  of  these  researches.  They  are  marked  throughout 
by  unflagging  industry,  by  novelty  and  acuteness  of  ob- 
servation, and  by  reasoning  power  of  a  high  and  varied 
kind.  These  pages  had  been  returned  "for  press"  when 
I  learned  that  the  relation  of  Ben  Nevis  and  his  colleagues 
to  the  vapor-laden  winds  of  the  Atlantic  had  not  escaped 
Mr.  Jamieson.  To  him  obviously  the  exploration  of  Loch- 
aber,  and  the  development  of  the  theory  of  the  Parallel 
Roads,  has  been  a  labor  of  love. 

Thus  ends  our  rapid  survey  of  this  brief  episode  in  the 
physical  history  of  the  Scottish  hills— brief,  that  is  to  say, 
in  comparison  with  the  immeasurable  lapses  of  time  through 
which,  to  produce  its  varied  structure  and  appearances,  our 
planet  must  have  passed.  In  the  survey  of  such  a  field 
two  things  are  specially  worthy  to  be  taken  into  account — 
the  widening  of  tbe  intellectual  horizon  and  the  reaction 
of  expanding  knowledge  upon  the  intellectual  organ  itself. 
At  first,  as  in  the  case  of  ancient  glaciers,  through  sheer 
want  of  capacity,  the  mind  refuses  to  take  in  revealed 
facts.  But  by  degrees  the  steady  contemplation  of  these 
facts  so  strengthens  and  expands  the  intellectual  powers 


THE   PARALLEL   ROADS   OF   GLEN  ROY  241 

that  where  truth  once  could  not  find  an  entrance  it  event- 
ually finds  a  home.* 


LITERATURE  OF  THE  SUBJECT 

Thomas  Pennant. — A  Tour  in  Scotland.     Vol.  iii.  17  76,  p.  394. 

John  MacCulloch. — On  the  Parallel  Roads  of  Glen  Roy.     Geol.  See.  Trans. 

vol.  iv.  1817,  p.  314. 
Thomas  Laudee  Dick  (afterward  Sib  Thomas  Dick-Laudee,  Bart.). — On  the 

Parallel  Roads  of  Lochaber.     Edin.  Roy.  Soc.  Trans.  1818,  vol.  ix.  p.*l. 
Charles  Daewin. — Observations  on  the  Parallel  Roads  of  Glen  Roy,  and  of 

the  other  parts  of  Lochaber  in  Scotland,  with  an  attempt  to  prove  that 

they  are  of  marine  origin.     Phil.  Trans.  1839,  voL  cxxix.  p.  39. 
Sib  Chables  Lybll. — Elements  of  Geology.     Second  edition,  1841. 
Louis  Agassiz. — ^The  Glacial  Theory  and  its  Recent  Progress — ^Parallel  Tot- 

races.     Edin.  New  Phil.  Journal,  1842,  voL  zxziii.  p.  236. 
David  Milne  (afterward  David  Milnb-Homb). — On  the  Paralld  Roads  of 

Lochaber;   with  Remarks  on  the  Change  of  Relative  Levels  of  Sea  and 

Land  in  Scotland,  and  on  the  Detrital  Deposits  in  that  Country.     Edin. 

Roy.  Soc.  Trans.  1847,  vol.  xvi.  p.  395. 
ROBEBT  Chambebs. — Ancient  Sea  Margins.     Edinburgh,  1848. 
H.  D.  RoQEES. — On  the  Parallel  Roads  of  Glen  Roy.     Royal  Inst.  Proceedings, 

1861,  vol.  iii.  p.  341. 
Thomas  F.  Jamieson. — On  the  Parallel  Roads  of  Glen  Roy,  and  their  Place  in 

the  History  of  the  Glacial  Period.     Quart.  Journal  GeoL  Soa  1863,  voL 

xix.  p.  235. 
Sib  Chables  Lyell. — Antiquity  of  Man.     1863,  p.  253. 
Rev.  R.  B.  Watson. — On  the  Marine  Origin  of  the  Parall^  Roads  of  Glen  Roy. 

Quart.  Joum.  Geol.  Soc.  1865,  vol.  xxii.  p.  9. 


*  The  formation,  connection,  successive  subsidence,  and  final  disappearance 
of  the  glacial  lakes  of  Lochaber  were  illustrated  in  the  discourse  here  reported 
by  the  model  just  described,  constructed  under  the  supervision  of  my  assistant, 
Mr.  John  Cottrell.  Glen  Gluoy  with  its  lake  and  road  and  the  cataract  over  its 
col;  Glen  Roy  and  its  three  roads  with  their  respective  cataracts  at  the  head  of 
Glen  Spey,  Glen  Glaster,  and  Glen  Spean,  were  all  represented.  The  succes- 
sive shiftings  of  the  barriers,  which  were  formed  of  plate  glass,  brought  each 
successive  lake  and  its  corresponding  road  into  view,  while  the  entire  removal 
of  the  barriers  caused  the  streams  to  flow  down  the  glens  of  the  model  as  they 
How  down  the  real  glens  of  to-day. 

Science 11 


242  FRAGMENTS   OF  SCIENCE 

Sir  John  Lubbock. — On  the  Parallel  Roads  of  Glen  Roy.     Quart.  Journ.  GeoL 

Soc.  1867,  vol.  xxiv.  p.  83. 
Charles  Babbage. — Observations  on  the  Parallel  Roads  of  Glen  Roy.     Quart. 

Joum.  Geol.  Soc.  1868,  vol.  xxiv.  p.  273. 
James  Nicol. — On  the  Origin  of  the  Parallel  Roads  of  Glen  Roy,  1869.     Geol. 

Soc.  Journal,  vol.  xxv.  p.  282. 
James  Niool. — How  the  Parallel  Roads  of  Glen  Roy  were  formed,  1872.    Geol. 

Soc.  Journal,  vol,  xxviii.  p.  237. 
Major-General  Sir  Henry  James,  R.E. — Notes  on  the  Parallel  Roads  of 

Lochaber.     4to.     1874. 


IX 

ALPINE  SCULPTURl 

TO  account  for  the  conformation  of  the  Alps,  two 
hypotheses  have  been  advanced,  which  may  be 
respectively  named  the  hypothesis  of  f  acture  and 
the  hypothesis  of  erosion.  The  former  assumes  that  the 
forces  by  which  the  mountains  were  elevated  produced 
fissures  in  the  earth's  crust,  and  that  the  valleys  of  the 
Alps  are  the  tracks  of  these  fissures;  while  the  latter  main- 
tains that  the  valleys  have  been  cut  out  by  the  action  of 
ice  and  water,  the  mountains  themselves  being  the  residual 
forms  of  this  grand  sculpture.  I  had  heard  the  Via  Mala 
cited  as  a  conspicuous  illustration  of  the  fissure  theory — 
the  profound  chasm  thus  named,  and  through  which  the 
Hinter-Rhein  now  flows,  could,  it  was  alleged,  be  nothing 
else  than  a  crack  in  the  earth's  crust.  To  the  Yia  Mala  I 
therefore  went,  in  1864,  to  instruct  myself  upon  the  point 
in  question. 

The  gorge  commences  about  a  quarter  of  an  hour  above 
Tusis;  and,  on  entering  it,  the  first  impression  certainly 
is  that  it  must  be  a  fissure.  This  conclusion  in  my  case 
was  modified  as  I  advanced.  Some  distance  up  the  gorge 
I  found  upon  the  slopes  to  my  right  quantities  of  rolled 
stones,  evidently  rounded  by  water-action.  Still  further 
up,  and  just  before  reaching  the  first  bridge  which  spans 
the  chasm,   I  found  more  rolled  stones,   associated   with 

(243) 


244  FRAGMENTS   OF  SCIENCE 

sand  and  gravel.  Through  this  mass  of  detritus,  fortu- 
nately, a  vertical  cutting  had  been  made,  which  exhibited 
a  section  showing  perfect  stratification.  There  was  no 
agency  in  the  place  to  roll  these  stones,  and  to  deposit 
these  alternating  layers  of  sand  and  pebbles,  but  the  river 
which  now  rushes  some  hundreds  of  feet  below  them.  At 
one  period  of  the  Yia  Mala's  history  the  river  must  have 
run  at  this  high  level.  Other  evidences  of  water-action 
soon  revealed  themselves.  From  the  parapet  of  the  first 
bridge  I  could  see  the  solid  rock  200  feet  above  the  bed 
of  the  river  scooped  and  eroded. 

It  is  stated  in  the  guide-books  that  the  river,  which 
usually  runs  along  the  bottom  of  the  gorge,  has  been 
known  almost  to  fill  it  during  violent  thunder-storms;  and 
it  may  be  urged  that  the  marks  of  erosion  which  the  sides 
of  the  chasm  exhibit  are  due  to  those  occasional  floods. 
In  reply  to  this,  it  may  be  stated  that  even  the  existence 
of  such  floods  is  not  well  authenticated,  and  that,  if  the 
supposition  were  true,  it  would  be  an  additional  argument 
in  favor  of  the  cutting  power  of  the  river.  For  if  floods 
operating  at  rare  intervals  could  thus  erode  the  rock,  the 
same  agency,  acting  without  ceasing  upon  the  river's  bed, 
must  certainly  be  competent  to  excavate  it. 

I  proceeded  upward,  and  from  a  point  near  another 
bridge  (which  of  them  I  did  not  note)  had  a  fine  view  of  a 
portion  of  the  gorge.  The  river  here  runs  at  the  bottom 
of  a  cleft  of  profound  depth,  but  so  narrow  that  it  might 
be  leaped  across.  That  this  cleft  must  be  a  crack  is  the 
impression  first  produced;  but  a  brief  inspection  suffices 
to  prove  that  it  has  been  cut  by  the  river.  From  top  to 
bottom  we  have  the  unmistakable  marks  of  erosion.  This 
cleft  was  best  seen  on  looking  downward  from  a  point  near 


ALPINE  SCULPTURE  245 

the  bridge;  but  looking  upward  from  the  bridge  itself,  the 
evidence  of  aqueous  erosion  was  equally  convincing. 

The  character  of  the  erosion  depends  upon  the  rock  as 
well  as  upon  the  river.  The  action  of  water  upon  some 
rocks  is  almost  purely  mechanical;  they  are  simply  ground 
away  or  detached  in  sensible  masses.  Water,  however,  in 
passing  over  limestone,  charges  itself  with  carbonate  of 
lime  without  damage  to  its  transparency;  the  rock  is  dis- 
solved in  the  water;  and  the  gorges  cut  by  water  in  such 
rocks  often  resemble  those  cut  in  the  ice  of  glaciers  by 
glacier  streams.  To  the  solubility  of  limestone  is  probably 
to  be  ascribed  the  fantastic  forms  which  peaks  of  this  rock 
usually  assume,  and  also  the  grottos  and  caverns  which 
interpenetrate  limestone  formations.  A  rock  capable  of 
being  thus  dissolved  will  expose  a  smooth  surface  after 
the  water  has  quitted  it;  and  in  the  case  of  the  Via  Mala 
it  is  the  polish  of  the  surfaces  and  the  curved  hollows 
scooped  in  the  sides  of  the  gorge  which  assure  us  that 
the  chasm  has  been  the  work  of  the  river. 

About  four  miles  from  Tusis,  and  not  far  from  the 
little  village  of  Zillis,  the  Via  Mala  opens  into  a  plain 
bounded  by  high  terraces.  It  occurred  to  me  the  moment 
I  saw  it  that  the  plain  had  been  the  bed  of  an  ancient 
lake;  and  a  farmer,  who  was  my  temporary  companion, 
immediately  informed  me  that  such  was  the  tradition  of 
the  neighborhood.  This  man  conversed  with  intelligence, 
and  as  I  drew  his  attention  to  the  rolled  stones,  which 
rest  not  only  above  the  river,  but  above  the  road,  and 
inferred  that  the  river  must  once  have  been  there  to  have 
rolled  those  stones,  he  saw  the  force  of  the  evidence  per- 
fectly. In  fact,  in  former  times,  and  subsequent  to  the 
retreat  of  the  great  glaciers,  a  rocky  barrier  crossed  the 


i46  FRAGMENTS   OF  SCIENCE 

valley  at  this  place,  damming  the  river  which  came  from 
the  mountains  higher  up.  A  lake  was  thus  formed  which 
poured  its  waters  over  the  barrier.  Two  actions  were  here 
at  work,  both  tending  to  obliterate  the  lake — the  raising 
of  its  bed  by  the  deposition  of  detritus,  and  the  cutting  of 
its  dam  by  the  river.  In  process  of  time  the  cut  deepened 
into  the  Via  Mala;  the  lake  was  drained,  and  the  river 
now  flows  in  a  definite  channel  through  the  plain  which 
its  waters  once  totally  covered. 

From  Tusis  I  crossed  to  Tiefenkasten  by  the  Schien 
Pass,  and  thence  over  the  Julier  Pass  to  Pontresina. 
There  are  three  or  four  ancient  lake-beds  between  Tie- 
fenkasten and  the  summit  of  the  Julier.  They  are  all  of 
the  same  type — a.  more  or  less  broad  and  level  valley- 
bottom,  with  a  barrier  in  front  through  which  the  river 
has  cut  a  passage,  the  drainage  of  the  lake  being  the 
consequence.  These  lakes  were  sometimes  dammed  by 
barriers  of  rock,  sometimes  by  the  moraines  of  ancient 
glaciers. 

An  example  of  this  latter  kind  occurs  in  the  Kosegg 
valley,  about  twenty  minutes  below  the  end  of  the  Kosegg 
glacier,  and  about  an  hour  from  Pontresina.  The  valley 
here  is  crossed  by  a  pine-covered  moraine  of  the  noblest 
dimensions;  in  the  neighborhood  of  London  it  might  be 
called  a  mountain.  That  it  is  a  moraine,  the  inspection 
of  it  from  a  point  on  the  Surlei  slopes  above  it  will  con- 
vince any  person  possessing  an  educated  eye.  Where, 
moreover,  the  interior  of  the  mound  is  exposed,  it  exhib- 
its moraine-matter — detritus  pulverized  by  the  ice,  with 
bowlders  entangled  in  it.  It  stretched  quite  across  the 
valley,  and  at  one  time  dammed  the  river  up.  But  now 
the  barrier  is  cut  through,  the  stream  having  about  one- 


ALPINE  SCULPTURE  247 

fourth  of  the  moraine  to  its  right,  and  the  remaining  three- 
fourths  to  its  left.  Other  moraines  of  a  more  resisting 
character  hold  their  ground  as  barriers  to  the  present 
day.  In  the  Yal  di  Campo,  for  example,  about  three- 
quarters  of  an  hour  from  Pisciadello,  there  is  a  moraine 
composed  of  large  bowlders,  which  interrupt  the  course 
of  a  river  and  compel  the  water  to  fall  over  them  in 
cascades.  They  have  in  great  part  resisted  its  action 
since  the  retreat  of  the  ancient  glacier  which  formed 
the  moraine.  Behind  the  moraine  is  a  lake-bed,  now 
converted  into  a  level  meadow,  which  rests  on  a  deep 
layer  of  mould. 

At  Pontresina  a  very  fine  and  instructive  gorge  is  to 
be  seen.  The  river  from  the  Morteratsch  glacier  rushes 
through  a  deep  and  narrow  chasm  which  is  spanned  at 
one  place  by  a  stone  bridge.  The  rock  is  not  of  a  char- 
acter to  preserve  smooth  polishing;  but  the  larger  features 
of  water-action  are  perfectly  evident  from  top  to  bottom. 
Those  features  are  in  part  visible  from  the  bridge,  but 
still  better  from  a  point  a  little  distance  from  the  bridge 
in  the  direction  of  the  upper  village  of  Pontresina.  The 
hollowing  out  of  the  rock  by  the  eddies  of  the  water  is 
here  quite  manifest.  A  few  minutes'  walk  upward  brings 
us  to  the  end  of  the  gorge;  and  behind  it  we  hare  the 
usual  indications  of  an  ancient  lake,  and  terraces  of  dis- 
tinct water  origin.  From  this  position  indeed  the  genesis 
of  the  gorge  is  clearly  revealed.  After  the  retreat  of  the 
ancient  glacier,  a  transverse  ridge  of  comparatively  resist- 
ing material  crossed  the  valley  at  this  place.  Over  the 
lowest  part  of  this  ridge  the  river  flowed,  rushing  steeply 
down  to  join  at  the  bottom  of  the  slope  the  stream  which 
issued  from  the  Kosegg  glacier.     On  this  incline  the  watel 


248  FRAGMENTS    OF  SCIENCE 

became  a  powerful  eroding  agent,  and  finally  cut  the 
channel  to  its  present  depth. 

Geological  writers  of  reputation  assume  at  this  place 
the  existence  of  a  fissure,  the  *' washing  out"  of  which 
resulted  in  the  formation  of  the  gorge.  Now,  no  exami- 
nation of  the  bed  of  the  river  ever  proved  the  existence 
of  this  fissure;  and  it  is  certain  that  water,  particularly 
when  charged  with  solid  matter  in  suspension,  can  cut  a 
channel  through  unfissured  rock.  Cases  of  deep  cutting 
can  be  pointed  out  where  the  clean  bed  of  the  stream  is 
exposed,  the  rock  which  forms  the  floor  of  the  river  not 
exhibiting  a  trace  of  fissure.  An  example  of  this  kind  on 
a  small  scale  occurs  near  the  Bernina  Gasthaus,  about  two 
hours  from  Pontresina.  A  little  way  below  the  junction 
of  the  two  streams  from  the  Bernina  Pass  and  the  Heuthal 
the  river  flows  through  a  channel  cut  by  itself,  and  20  or 
80  feet  in  depth.  At  some  places  the  river-bed  is  covered 
with  rolled  stones;  at  other  places  it  is  bare,  but  shows  no 
trace  of  fissure.  The  abstract  power  of  water,  if  I  may 
use  the  term,  to  cut  through  rock  is  demonstrated  by  such 
instances.  But  if  water  be  competent  to  form  a  gorge 
without  the  aid  of  a  fissure,  why  assume  the  existence  of 
such  fissures  in  cases  like  that  at  Pontresina?  It  seems 
far  more  philosophical  to  accept  the  simple  and  impressive 
history  written  on  the  walls  ot  those  gorges  by  the  agent 
which  produced  them. 

Numerous  cases  might  be  pointed  out,  varying  in  mag- 
nitude, but  all  identical  in  kind,  of  barriers  which  crossed 
valleys  and  formed  lakes  having  been  cut  through  by 
rivers,  narrow  gorges  being  the  consequence.  One  of  the 
most  famous  examples  of  this  kind  is  the  Finsteraarschlucht 
in  the  valley  of  Hasli.     Here  the  ridge  called  the  Kirchet 


ALPINE  SCULPTURE  249 

seems  split  across,  and  the  river  Aar  rushes  through  the 
fissure.  Behind  the  barrier  we  have  the  meadows  and 
pastures  of  Imhof  resting  on  the  sediment  of  an  ancient 
lake.  Were  this  an  isolated  case,  one  might,  with  an  appar- 
ent show  of  reason,  conclude  that  the  Finsteraarschlucht 
was  produced  by  an  earthquake,  as  some  suppose  it  to  have 
been ;  but  when  we  find  it  to  be  a  single  sample  of  actions 
which  are  frequent  in  the  Alps — when  probably  a  hundred 
cases  of  the  same  kind,  though  different  in  magnitude,  can 
be  pointed  out — it  seems  quite  unphilosophical  to  assume 
that  in  each  particular  case  a.ii  earthquake  was  at  hand  to 
form  a  channel  for  the  river.  As  in  the  case  of  the  bar- 
rier at  Pontresina,  the  Kirchet,  after  the  retreat  of  the  Aar 
glacier,  dammed  the  waters  flowing  from  it,  thus  forming 
a  lake,  on  tae  bed  of  which  now  stands  the  village  of 
Imhof.  Over  this  ]:)arrier  the  Aar  tumbled  toward  Mey- 
ringen,  cutting,  as  the  centuries  passed,  its  bed  ever  deeper, 
until  finally  it  became  deep  enough  to  drain  the  lake,  leav- 
ing in  its  place  the  alluvial  plain,  through  which  the  river 
now  flows  in  a  definite  channel. 

In  1866  I  subjected  the  Finsteraarschlucht  to  a  close 
examination.  The  earthquake  theory  already  adverted  to 
was  then  prevalent  regarding  it,  and  I  wished  to  see 
whether  any  evidences  existed  of  aqueous  erosion.  Near 
the  summit  of  the  Kirchet  is  a  signboard  inviting  the 
traveller  to  visit  the  Aarenschlucht^  a  narrow  lateral  gorge 
which  runs  down  to  the  very  bottom  of  the  principal  one. 
The  aspect  of  this  smaller  chasm  from  bottom  to  top 
proves  to  demonstration  that  water  had  in  former  ages 
been  there  at  work.  It  is  scooped,  rounded,  and  pol- 
ished, so  as  to  render  palpable  to  the  most  careless  eye 
that  it  is  a  gorge  of  erosion.      But  it  was  regarding  the 


250  FRAGMENTS    OF  SCIENCE 

sides  of  tlie  great  cliasm  that  instruction  was  needed,  and 
from  its  edge  nothing  to  satisfy  me  could  be  seen.  1 
therefore  stripped  and  waded  into  the  river  until  a  point 
was  reached  which  commanded  an  excellent  view  of  both 
sides  of  the  gorge.  The  water  was  cutting  cold,  but  I  was 
repaid.  Below  me  on  the  left-hand  side  was  a  jutting  cliff 
which  bore  the  thrust  of  the  river  and  caused  the  Aar  to 
swerve  from  its  direct  course.  From  top  to  bottom  this 
cliff  was  polished,  rounded,  and  scooped.  There  was  no 
room  for  doubt.  The  river  which  now  runs  so  deeply 
down  had  once  been  above.  It  has  been  the  delver  of  its 
own  channel  through  the  barrier  of  the  Kirchet. 

But  the  broad  view  taken  by  the  advocates  of  the  fract- 
ure theory  is,  that  the  valleys  themselves  follow  the  tracks 
of  primeval  fissures  produced  by  the  upheaval  of  the  land, 
the  cracks  across  the  barriers  referred  to  being  in  reality 
portions  of  the  great  cracks  which  formed  the  valleys. 
Such  an  argument,  however,  would  virtually  concede  the 
theory  of  erosion  as  applied  to  the  valleys  of  the  Alps. 
The  narrow  gorges,  often  not  more  than  twenty  or  thirty 
feet  across,  sometimes  even  narrower,  frequently  occur  at 
the  bottom  of  broad  valleys.  Such  fissures  might  enter 
into  the  list  of  accidents  which  gave  direction  to  the  real 
erosive  agents  which  scooped  the  valley  out;  but  the  for- 
mation of  the  valley,  as  it  now  exists,  could  no  more  be 
ascribed  to  such  cracks  than  the  motion  of  a  railway  train 
could  be  ascribed  to  the  finger  of  the  engineer  which  turns 
on  the  steam. 

These  deep  gorges  occur,  I  believe,  for  the  most  part  in 
limestone  strata;  and  the  effects  which  the  merest  driblet 
of  water  can  produce  on  limestone  are  quite  astonishing. 
It  is  not  uncommon  to  meet  chasms  of  considerable  depth 


ALPINE  SCULPTURE  261 

produced  by  small  streams  the  beds  of  which  are  dry  for 
a  large  portion  of  the  year.  Right  and  left  of  the  larger 
gorges  such'  secondary  chasms  are  often  found.  The  idea 
of  time  must,  I  think,  be  more  and  more  included  in  our 
reasonings  on  these  phenomena.  Happily,  the  marks  which 
the  rivers  have,  in  most  cases,  left  behind  them,  and  which 
refer,  geologically  considered,  to  actions  of  yesterday,  give 
us  ground  and  courage  to  conceive .  what  may  be  effected 
in  geologic  periods.  Thus  the  modern  portion  of  the  Via 
Mala  throws  light  upon  the  whole.  Near  Bergiin,  in  the 
valley  of  the  Albula,  there  is  also  a  little  Via  Mala,  which 
is  not  less  significant  than  the  great  one.  The  river  flows 
here  through  a  profound  limestone  gorge,  and  to  the  very 
edges  of  the  gorge  we  have  the  evidences  of  erosion.  But 
the  most  striking  illustration  of  water-action  upon  limestone 
rock  that  I  have  ever  seen  is  the  gorge  at  Pfaffers.  Here 
the  traveller  passes  along  the  side  of  the  chasm  midway 
between  top  and  bottom.  Whichever  way  he  looks,  back- 
ward or  forward,  upward  or  downward,  toward  the  sky  or 
toward  the  river,  he  meets  everywhere  the  irresistible  and 
impressive  evidence  that  this  wonderful  fissure  has  been 
sawn  through  the  mountain  by  the  waters  of  the  Tamina. 
I  have  thus  far  confined  myself  to  the  consideration  of 
the  gorges  formed  by  the  cutting  through  of  the  rock- 
barriers  which  frequently  cross  the  valleys  of  the  Alps; 
as  far  as  they  have  been  examined  by  me  they  are  the 
work  of  erosion.  But  the  larger  question  still  remains.  To 
what  action  are  we  to  ascribe  the  formation  of  the  valleys 
themselves?  This  question  includes  that  of  the  formation 
of  the  mountain -ridges,  for  were  the  valleys  wholly  filled 
the  ridges  would  disappear.  Possibly  no  answer  can  be 
given  to  this  question  which  is  not  beset  with  more  or  less 


250  FRAGMENTS    OF   SCIENCE 

sides  of  the  great  cliasm  that  instruction  was  needed,  and 
from  its  edge  nothing  to  satisfy  me  could  be  seen.  1 
therefore  stripped  and  waded  into  the  river  until  a  point 
was  reached  which  commanded  an  excellent  view  of  both 
sides  of  the  gorge.  The  water  was  cutting  cold,  but  I  was 
repaid.  Below  me  on  the  left-hand  side  was  a  jutting  cliff 
which  bore  the  thrust  of  the  river  and  caused  the  Aar  to 
swerve  from  its  direct  course.  From  top  to  bottom  this 
cliff  was  polished,  rounded,  and  scooped.  There  was  no 
room  for  doubt.  The  river  which  now  runs  so  deeply 
down  had  once  been  above.  It  has  been  the  delver  of  its 
own  channel  through  the  barrier  of  the  Kirchet. 

But  the  broad  view  taken  by  the  advocates  of  the  fract- 
ure theory  is,  that  the  valleys  themselves  follow  the  tracks 
of  primeval  fissures  produced  by  the  upheaval  of  the  land, 
the  cracks  across  the  barriers  referred  to  being  in  reality 
portions  of  the  great  cracks  which  formed  the  valleys. 
Such  an  argument,  however,  would  virtually  concede  the 
theory  of  erosion  as  applied  to  the  valleys  of  the  Alps. 
The  narrow  gorges,  often  not  more  than  twenty  or  thirty 
feet  across,  sometimes  even  narrower,  frequently  occur  at 
the  bottom  of  broad  valleys.  Such  fissures  might  enter 
into  the  list  of  accidents  which  gave  direction  to  the  real 
erosive  agents  which  scooped  the  valley  out;  but  the  for- 
mation of  the  valley,  as  it  now  exists,  could  no  more  be 
ascribed  to  such  cracks  than  the  motion  of  a  railway  train 
could  be  ascribed  to  the  finger  of  the  engineer  which  turns 
on  the  steam. 

These  deep  gorges  occur,  I  believe,  for  the  most  part  in 
limestone  strata;  and  the  effects  which  the  merest  driblet 
of  water  can  produce  on  limestone  are  quite  astonishing. 
It  is  not  uncommon  to  meet  chasms  of  considerable  depth 


ALPINE  SCULPTURE  251 

produced  by  small  streams  the  beds  of  which  are  dry  for 
a  large  portion  of  the  year.  Eight  and  left  of  the  larger 
gorges  snch'  secondary  chasms  are  often  found.  The  idea 
of  time  must,  I  think,  be  more  and  more  included  in  our 
reasonings  on  these  phenomena.  Happily,  the  marks  which 
the  rivers  have,  in  most  cases,  left  behind  them,  and  which 
refer,  geologically  considered,  to  actions  of  yesterday,  give 
■us  ground  and  courage  to  conceive .  what  may  be  effected 
in  geologic  periods.  Thus  the  modern  portion  of  the  Yia 
Mala  throws  light  upon  the  whole.  Near  Bergiin,  in  the 
valley  of  the  Albula,  there  is  also  a  little  Via  Mala,  which 
is  not  less  significant  than  the  great  one.  The  river  flows 
here  through  a  profound  limestone  gorge,  and  to  the  very 
edges  of  the  gorge  we  have  the  evidences  of  erosion.  But 
the  most  striking  illustration  of  water-action  npon  limestone 
rock  that  I  have  ever  seen  is  the  gorge  at  Pfaffers.  Here 
the  traveller  passes  along  the  side  of  the  chasm  midway 
between  top  and  bottom.  Whichever  way  he  looks,  back- 
ward or  forward,  upward  or  downward,  toward  the  sky  or 
toward  the  river,  he  meets  everywhere  the  irresistible  and 
impressive  evidence  that  this  wonderful  fissure  has  been 
sawn  through  the  mountain  by  the  waters  of  the  Tamina. 
I  have  thus  far  confined  myself  to  the  consideration  of 
the  gorges  formed  by  the  cutting  through  of  the  rock- 
barriers  which  frequently  cross  the  valleys  of  the  Alps; 
as  far  as  they  have  been  examined  by  me  they  are  the 
work  of  erosion.  But  the  larger  question  still  remains,  To 
what  action  are  we  to  ascribe  the  formation  of  the  valleys 
themselves?  This  question  includes  that  of  the  formation 
of  the  mountain -ridges,  for  were  the  valleys  wholly  filled 
the  ridges  would  disappear.  Possibly  no  answer  can  be 
given  to  this  question  which  is  not  beset  with  more  or  lesa 


252  FRAGMENTS   OF  SCIENCE 

of  difficulty.  Special  localities  might  be  found  whicli  would 
seem  to  contradict  every  solution  wHch  refers  the  confor- 
mation of  the  Alps  to  the  operation  of  a  single  cause. 

Still  the  Alps  present  features  of  a  character  sufficiently 
definite  to  bring  the  question  of  their  origin  within  the 
sphere  of  close  reasoning.  That  they  were  in  whole  or  in 
part  once  beneath  the  sea  will  not  be  disputed;  for  they 
are  in  great  part  composed  of  sedimentary  rocks  which  re- 
quired a  sea  to  form  them.  Their  present  elevation  above 
the  sea  is  due  to  one  of  those  local  changes  in  the  shape 
of  the  earth  which  have  been  of  frequent  occurrence 
throughout  geologic  time,  in  some  cases  depressing  the 
land,  and  in  others  causing  the  sea-bottom  to  protrude  be- 
yond its  surface.  Considering  the  inelastic  character  of 
its  materials,  the  protuberance  of  the  Alps  could  hardly 
have  been  pushed  out  without  dislocation  and  fracture; 
and  this  conclusion  gains  in  probability  when  we  consider 
the  foldings,  contortions,  and  even  reversals  in  position  of 
the  strata  in  many  parts  of  the  Alps.  Such  changes  in  the 
position  of  beds,  which  were  once  horizontal,  could  not 
have  been  effected  without  dislocation.  Fissures  would  be 
produced  by  these  changes;  and  such  fissures,  the  advo- 
cates of  the  fracture  theory  contend,  mark  the  positions 
of  the  valleys  of  the  Alps. 

Imagination  is  necessary  to  the  man  of  science,  and  we 
could  not  reason  on  our  present  subject  without  the  power 
of  presenting  mentally  a  picture  of  the  earth's  crust  cracked 
and  fissured  by  the  forces  which  produced  its  upheaval. 
Imagination,  however,  must  be  strictly  checked  by  reason 
and  by  observation.  That  fractures  occurred  cannot,  I 
think,  be  doubted,  but  that  the  valleys  of  the  Alps  are 
thus  formed  is  a  conclusion  not  at  all  involved  in  the  ad- 


ALPINE   SCULPTURE  253 

mission  of  dislocations.  I  never  met  witli  a  precise  state- 
ment of  the  manner  in  which  the  advocates  of  the  fissure 
theory  suppose  the  forces  to  have  acted — whether  they  as- 
sume a  general  elevation  of  the  region,  or  a  local  elevation 
of  distinct  ridges ;  or  whether  they  assume  local  subsidences 
after  a  general  elevation,  or  whether  they  would  superpose 
upon  the  general  upheaval  minor  and  local  upheavals. 

In  the  absence  of  any  distinct  statement,  I  will  assume 
the  elevation  to  be  general — that  a  swelling  out  of  the 
earth's  crust  occurred  here,  sufficient  to  place  the  most 
prominent  portions  of  the  protuberance  three  miles  above 
the  sea- level.  To  fix  the  ideas,  let  us  consider  a  circular 
portion  of  the  crust,  say  one  hundred  miles  in  diameter, 
and  let  us  suppose,  in  the  first  instance,  the  circumference 
of  this  circle  to  remain  fixed,  and  that  the  elevation  was 
confined  to  the  space  within  it.  The  upheaval  would  throw 
the  crust  into  a  state  of  strain;  and,  if  it  were  inflexible, 
the  strain  must  be  relieved  by  fracture.  Crevasses  would 
thus  intersect  the  crust.  Let  us  now  inquire  what  propor- 
tion the  area  of  these  open  fissures  is  likely  to  bear  to  the 
area  of  the  unfissured  crust.  An  approximate  answer  is 
all  that  is  here  required;  for  the  problem  is  of  such  a 
character  as  to  render  minute  precision  unnecessary. 

No  one,  I  think,  would  affirm  that  the  area  of  the  fis- 
sures would  be  one- hundredth  the  area  of  the  land.  For 
let  us  consider  the  strain  upon  a  single  line  drawn  over 
the  summit  of  the  protuberance  from  a  point  on  its  rim 
to  a  point  opposite.  Kegarding  the  protuberance  as  a 
spherical  swelling,  the  length  of  the  arc  corresponding  to 
a  chord  of  100  miles  and  a  versed  sine  of  3  miles  is  100*24: 
miles;  consequently,  the  surface  to  reach  its  new  position 
must  stretch  0-24  of  a  mile,  or  be  broken.     A  fissure  or  a 


254  FRAGMENTS   OF  SCIENCE 

number  of  cracks  with  this  total  width  would  relieve  the 
strain;  that  is  to  say,  the  sum  of  the  widths  of  all  the 
cracks  over  the  length  of  100  miles  would  be  420  yards. 
If,  instead  of  comparing  the  width  of  the  fissures  with  the 
length  of  the  lines  of  tension,  we  compared  their  areas 
with  the  area  of  the  unfissured  land,  we  should  of  course 
find  the  proportion  much  less.  These  considerations  will 
help  the  imagination  to  realize  what  a  small  ratio  the  area 
of  the  open  fissures  must  bear  to  the  unfissured  crust. 
They  enable  us  to  say,  for  example,  that  to  assume  the 
area  of  the  fissures  to  be  one-tenth  of  the  area  of  the  land 
would  be  quite  absurd,  while  that  the  area  of  the  fissures 
could  be  one-half  or  more  than  one-half  that  of  the  land 
would  be  in  a  proportionate  degree  unthinkable.  If  we 
suppose  the  elevation  to  be  due  to  the  shrinking  or  subsi- 
dence of  the  land  all  round  our  assumed  circle,  we  arrive 
equally  at  the  conclusion  that  the  area  of  the  open  fissures 
would  be  altogether  insignificant  as  compared  with  that  of 
the  unfissured  crust. 

To  those  who  have  seen  them  from  a  commanding  ele- 
vation, it  is  needless  to  say  that  the  Alps  themselves  bear 
no  sort  of  resemblance  to  the  picture  which  this  theory  pre- 
sents to  us.  Instead  of  deep  cracks  with  approximately 
vertical  walls,  we  have  ridges  running  into  peaks,  and 
gradually  sloping  to  form  valleys.  Instead  of  a  fissured 
crust,  we  have  a  state  of  things  closely  resembling  the 
surface  of  the  ocean  when  agitated  by  a  storm.  The  val- 
leys, instead  of  being  much  narrower  than  the  ridges,  oc- 
cupy the  greater  space.  A  plaster  cast  of  the  Alps  turned 
npside  down,  so  as  to  invert  the  elevations  and  depres- 
eions,  would  exhibit  blunter  and  broader  mountains,  with 
narrower  valleys   between   them,   than   the  present   ones. 


ALPINE   SCULPTURE  255 

The  valleys  that  exist  cannot,  I  think,  with  any  correct- 
ness of  language,  be  called  fissures.  It  may  be  urged  that 
they  originated  in  fissures:  but  even  this  is  unproved,  and, 
were  it  proved,  the  fissures  would  still  play  the  subordinate 
part  of  giving  direction  to  the  agents  which  are  to  be  re- 
garded as  the  real  sculptors  of  the  Alps. 

The  fracture  theory,  then,  if  it  regards  the  elevation  of 
the  Alps  as  due  to  the  operation  of  a  force  acting  through- 
out the  entire  region,  is,  in  my  opinion,  utterly  incompe- 
tent to  account  for  the  conformation  of  the  country.  If, 
on  the  other  hand,  we  are  compelled  to  resort  to  local  dis- 
turbances, the  manipulation  of  the  earth's  crust  necessary 
to  obtain  the  valleys  and  the  mountains  will,  I  imagine, 
bring  the  difficulties  of  the  theory  into  very  strong  relief. 
Indeed,  an  examination  of  the  region  from  many  of  the 
more  accessible  eminences — from  the  Gralenstock,  the  Grau- 
haupt,  the  Pitz  Languard,  the  Monte  Confinale — or,  better 
still,  from  Mont  Blanc,  Monte  Eosa,  the  Jungfrau,  the 
Finsteraarhorn,  the  Weisshorn,  or  the  Matterhorn,  where 
local  peculiarities  are  toned  down,  and  the  operations  of 
the  powers  which  really  made  this  region  what  it  is  are 
alone  brought  into  prominence-r-must,  I  imagine,  convince 
every  physical  geologist  of  the  inability  of  any  fracture 
theory  to  account  for  the  present  conformation  of  the  Alps. 

A  correct  model  of  the  mountains,  with  an  unexagger- 
ated  vertical  scale,  produces  the  same  effect  upon  the  mind 
as  the  prospect  from  one  of  the  highest  peaks.  We  are 
apt  to  be  influenced  by  local  phenomena  which,  though, 
insignificant  in  view  of  the  general  question  of  Alpine  con- 
formation, are,  with  reference  to  our  customary  standards, 
vast  and  impressive.  In  a  true  model  those  local  peculi- 
arities disappear;  for  on  the  scale  of  a  model  they  are  too 


256  FRAGMENTS   OF  SCIENCE 

email  to  be  visible;  while  the  essential  facts  and  forms  are 
presented  to  the  undistracted  attention. 

A  minute  analysis  of  the  phenomena  strengthens  the 
conviction  which  the  general  aspect  of  the  Alps  fixes  in 
the  mind.  We  find,  for  example,  numerous  valleys  which 
the  most  ardent  plutonist  would  not  think  of  ascribing  to 
any  other  agency  than  erosion.  That  such  is  their  genesis 
and  history  is  as  certain  as  that  erosion  produced  the 
Chines  in  the  Isle  of  Wight.  From  these  indubitable 
cases  of  erosion — commencing,  if  necessary,  with  the  small 
ravines  which  run  down  the  flanks  of  the  ridges,  with  their 
little  working  navigators  at  their  bottoms — we  can  proceed, 
by  almost  insensible  gradations,  to  the  largest  valleys  of 
the  Alps;  and  it  would  perplex  the  plutonist  to  ^il  upon 
the  point  at  which  fracture  begins  to  play  a  material  part. 

In  ascending  one  of  the  larger  valleys,  we  enter  it  whei>e 
it  is  wide  and  where  the  eminences  are  gentle  on  either 
side.  The  flanking  mountains  become  higher  and  more 
abrupt  as  we  ascend,  and  at  length  we  reach  a  place  where 
the  depth  of  the  valley  is  a  maximum.  Continuing  our 
walk  upward,  we  find  ourselves  flanked  by  gentler  slopes, 
and  finally  emerge  from  the  valley  and  reach  the  summit 
of  an  open  col,  or  depression  in  the  chain  of  mountains. 
This  is  the  common  character  of  the  large  valleys.  Cross- 
ing the  col,  we  descend  along  the  opposite  slope  of  the 
chain,  and  through  the  same  series  of  appearances  in  the 
reverse  order.  If  the  valleys  on  both  sides  of  the  col  were 
produced  by  fissures,  what  prevents  the  fissure  from  pro- 
longing itself  across  the  col  ?  The  case  here  cited  is  repre- 
sentative; and  I  am  not  acquainted  with  a  single  instance 
in  the  Alps  where  the  chain  has  been  cracked  in  the  man- 
ner indicated.     The  cols  are  simply  depressions;   in  many 


ALPINE   SCULPTURE  257 

of  whicli  tlie  unfissured  rock  can  be  traced  from  side  to 
side. 

The  typical  instance  just  sketched  follows  as  a  natural 
consequence  from  the  theory  of  erosion.  Before  either  ice 
or  water  can  exert  great  power  as  an  erosive  agent,  it  must 
collect  in  sufficient  mass.  On  the  higher  slopes  and  pla- 
teaus— in  the  region  of  cols — the  power  is  not  fully  devel- 
oped; but  lower  down  tributaries  unite,  erosion  is  carried 
on  with  increased  vigor,  and  the  excavation  gradually 
reaches  a  maximum.  Lower  still  the  elevations  diminish 
and  the  slopes  become  more  gentle;  the  cutting  power 
gradually  relaxes,  until  finally  the  eroding  agent  quits  the 
mountains  altogether,  and  the  grand  effects  which  it  pro- 
duced in  the  earlier  portions  of  its  course  entirely  dis- 
appear. 

I  have  hitherto  confined  myself  to  the  consideration 
of  the  broad  question  of  the  erosion  theory  as  compared 
with  the  fracture  theory;  and  all  that  I  have  been  able  to 
observe  and  think  with  reference  to  the  subject  leads  me 
to  adopt  the  former.  Under  the  term  erosion  I  include 
the  action  of  water,  of  ice,  and  of  the  atmosphere,  including 
frost  and  rain.  Water  and  ice,  however,  are  the  principal 
agents,  and  which  of  these  two  has  produced  the  greatest 
effect  it  is  perhaps  impossible  to  say.  Two  years  ago  I 
wrote  a  brief  note  "On  the  Conformation  of  the  Alps,"* 
in  which  I  ascribed  the  paramount  influence  to  glaciers. 
The  facts  on  which  that  opinion  was  founded  are,  I  think, 
unassailable;  but  whether  the  conclusion  then  announced 
fairly  follows  from  the  facts  is,  I  confess,  an  open  ques- 
tion. 

The  arguments  which  have  been  thus  far  urged  against 

»  Phil  Mag.,  vol.  xxiv.  p.  169. 


258  FRAGMENTS   OF  SCIENCE 

the  conclusion  are  not  convincing.  Indeed,  the  idea  of 
glacier  erosion  appears  so  daring  to  some  minds  that  its 
boldness  alone  is  deemed  its  sufficient  refutation.  It  is, 
however,  to  be  remembered  that  a  precisely  similar  posi- 
tion was  taken  up  by  many  excellent  workers  when  the 
question  of  ancient  glacier  extension  was  first  mooted.  The 
idea  was  considered  too  hardy  to  be  entertained;  and  the 
evidences  of  glacial  action  were  sought  to  be  explained  by 
reference  to  almost  any  process  rather  than  the  true  one. 
Let  those  who  so  wisely  took  the  side  of  "boldness"  in 
that  discussion  beware  lest  they  place  themselves,  with  ref- 
erence to  the  question  of  glacier  erosion,  in  the  position 
formerly  occupied  by  their  opponents. 

Looking  at  the  little  glaciers  of  the  present  day — mere 
pygmies  as  compared  to  the  giants  of  the  glacial  epoch — 
we  find  that  from  every  one  of  them  issues  a  river  more 
or  less  voluminous,  charged  with  the  matter  which  the  ice 
has  rubbed  from  the  rocks.  Where  the  rocks  are  soft,  the 
amount  of  this  finely  pulverized  matter  suspended  in  the 
water  is  very  great.  The  water,  for  example,  of  the  river 
which  flows  from  Santa  Catarina  to  Bormio  is  thick  with 
it.  The  Khine  is  charged  with  this  matter,  and  by  it  has 
so  silted  up  the  Lake  of  Constance  as  to  abolish  it  for  a 
large  fraction  of  its  length.  The  Khone  is  charged  with 
it,  and  tens  of  thousands  of  acres  of  cultivable  land  are 
formed  by  the  silt  above  the  Lake  of  Geneva. 

In  the  case  of  every  glacier  we  have  two  agents  at  work 
— the  ice  exerting  a  crushing  force  on  every  point  of  its 
bed  which  bears  its  weight,  and  either  rasping  this  point 
into  powder  or  tearing  it  bodily  from  the  rock  to  which 
it  belongs;  while  the  water  which  everywhere  circulates 
upon  the  bed  of  the  glacier  continually  washes  the  detritus 


ALPINE  SCULPTURE  259 

away  and  leaves  the  rock  clean  for  further  abrasion.  Con- 
fining the  action  of  glaciers  to  the  simple  rubbing  away  of 
the  rocks,  and  allowing  them  sufficient  time  to  act,  it  is  not 
a  matter  of  opinion,  but  a  physical  certainty,  that  they  will 
scoop  out  valleys.  But  the  glacier  does  more  than  abrade. 
Eocks  are  not  homogeneous;  they  are  intersected  by  joints 
and  places  of  weakness,  which  divide  them  into  virtually 
detached  masses.  A  glacier  is  undoubtedly  competent  to 
root  such  masses  bodily  away.  Indeed,  the  mere  d  priori 
consideration  of  the  subject  proves  the  competence  of  a 
glacier  to  deepen  its  bed.  Taking  the  case  of  a  glacier 
1,000  feet  deep  (and  some  of  the  older  ones  were  probably 
three  times  this  depth),  and  allowing  40  feet  of  ice  to  an 
atmosphere,  we  find  that  on  every  square  inch  of  its  bed 
such  a  glacier  presses  with  a  weight  of  375  pounds,  and 
on  every  square  yard  of  its  bed  with  a  weight  of  486,000 
pounds.  With  a  vertical  pressure  of  this  amount  the  gla- 
cier is  urged  down  its  valley  by  the  pressure  from  behind. 
We  can  hardly,  I  think,  deny  to  such  a  tool  a  power 
of  excavation. 

The  retardation  of  a  glacier  by  its  bed  has  been  referred 
to  as  proving  its  impotence  as  an  erosive  agent;  but  this 
very  retardation  is  in  some  measure  an  expression  of  the 
magnitude  of  the  erosive  energy.  Either  the  bed  must 
give  way  or  the  ice  must  slide  over  itself.  We  get  indeed 
some  idea  of  the  crushing  pressure  which  the  moving  gla- 
cier exercises  against  its  bed  from  the  fact  that  the  resist- 
ance, and  the  effort  to  overcome  it,  are  such  as  to  make 
the  upper  layers  of  a  glacier  move  bodily  over  the  lower 
ones — a  portion  only  of  the  total  motion  being  due  to  the 
progress  of  the  entire  mass  of  the  glacier  down  its  valley. 

The  sudden  bend  in  the  valley  of  the  Khone  at  Martigny 


260  FRAGMENTS    OF  SCIENCE 

lias  also  been  regarded  as  conclusive  evidence  against  tlie 
theory  of  erosion.  "Why,"  it  has  been  asked,  "did  not 
the  glacier  of  the  Khone  go  straight  forward  instead  of 
making  this  awkward  bend?"  But  if  the  valley  be  a 
crack,  why  did  the  crack  make  this  bend?  The  crack, 
I  submit,  had  at  least  as  much  reason  to  prolong  itself  in  a 
straight  line  as  the  glacier  had.  A  statement  of  Sir  John 
Herschel  with  reference  to  another  matter  is  perfectly  ap- 
plicable here:  "A  crack  once  produced  has  a  tendency  to 
run — for  this  plain  reason,  that  at  its  momentary  limit,  at 
the  point  at  which  it  has  just  arrived,  the  divellent  force 
on  the  molecules  there  situated  is  counteracted  x>nly  by 
half  of  the  cohesive  force  which  acted  when  there  was  no 
crack,  viz.,  the  cohesion  of  the  uncracked  portion  alone" 
("Proc.  Eoy.  Soc,"  vol.  xii.  p.  678).  To  account,  then, 
for  the  bend,  the  adherent  of  the  fracture  theory  must 
assume  the  existence  of  some  accident  which  turned  the 
crack  at  right  angles  to  itself;  and  he  surely  will  permit 
the  adherent  of  the  erosion  theory  to  make  a  similar 
assumption. 

The  influence  of  small  accidents  on  the  direction  of 
rivers  is  beautifully  illustrated  in  glacier  streams,  which 
are  made  to  cut  either  straight  or  sinuous  channels  by 
causes  apparently  of  the  most  trivial  character.  In  his  in- 
teresting paper  ' '  On  the  Lakes  of  Switzerland, ' '  M.  Studer 
also  refers  to  the  bend  of  the  Ehine  at  Sargans  in  proof 
that  the  river  must  there  follow  a  pre-existing  fissure.  I 
made  a  special  expedition  to  the  place  in  1864;  and,  though 
it  was  plain  that  M.  Studer  had  good  grounds  for  the  se- 
lection of  this  spot,  I  was  unable  to  arrive  at  his  conclusion 
as  to  the  necessity  of  a  fissure. 

Again,  in  the  interesting  volume  recently  published  by 


ALPINE  SCULPTURE 


261 


the  Swiss  Alpine  Club,  M.  Desor  informs  us  that  tlie  Swiss 
naturalists  who  met  last  year  at  Samaden  visited  the  end 
of  the  Morteratsch  glacier,  and  there  convinced  themselves 
that  a  glacier  had  no  tendency  whatever  to  imbed  itself 
in  the  soil.  I  scarcely  think  that  the  question  of  glacier 
erosion,  as  applied  either  to  lakes  or  valleys,  is  to  be  dis- 
posed of  so  easily.  Let  me  record  here  my  experience  of 
the  Morteratsch  glacier.  I  took  with  me,  in  1864,  a  theod- 
olite to  Pontresina,  and  while  there  had  to  congratulate 
myself  on  the  aid  of  my  friend  Mr.  Hirst,  who,  in  1867, 
did  such  good  service  upon  the  Mer  de  Glace  and  its  trib- 
utaries. We  set  out  three  lines  across  the  Morteratsch 
glacier,  one  of  which  crossed  the  ice -stream  near  the  well- 
known  hut  of  the  painter  Georgei,  while  the  two  others 
were  staked  out,  the  one  above  the  hut  and  the  other  below 
it.  Calling  the  highest  line  A,  the  line  which  crossed  the 
glacier  at  the  hut  B,  and  the  lowest  line  C,  the  following 
are  the  mean  hourly  motions  of  the  three  lines,  deduced 
from  observations  which  extended  over  several  days.  On 
each  line  eleven  stakes  were  fixed,  which  are  designated 
by  the  figures  1,  2,  3,  etc.,  in  the  Tables. 


Morteratsch  Glacier,  Line  A 


Vo.  of  Stake 

1  . 

2  . 

3  . 

4  . 

5  . 

6  . 
1  , 

8  . 

9  . 

10  . 

11  . 


Hourly  Motion 

0-35 

inch 

0-49 

0-53 

0-64 

0-56 

0-54 

0-52 

0-49 

0-40 

0-29 

t< 

0-20 

te 

262  FRAGMENTS   OF  SCIENCE 

As  in  all  other  measurements  of  this  kind,  the  retarding 
influence  of  the  sides  of  the  glacier  is  manifest:  the  centre 
moves  with  the  greatest  velocity. 

Morteratsch  Glacier,  Line  B 

No.  of  Stake  Hourly  Motion 

1 0-05  inch 

2 0-14  " 

3 0-24  " 

4  .                                                ....  0-32  " 

5 0-41  " 

6 0-44  " 

1 0-44  " 

8 0-45  " 

9 0-43  '* 

10 0-44  " 

11 0-44  " 

The  first  stake  of  this  line  was  quite  close  to  the  edge 
of  the  glacier,  and  the  ice  was  thin  at  the  place,  hence  its 
slow  motion.  Crevasses  prevented  us  from  carrying  the 
line  sufficiently  far  across  to  render  the  retardation  of  the 
further  side  of  the  glacier  fully  evident. 

Morteratsch  Glacier,  Line  C 
No.  of  Stake  Hourly  Motion 

1  .         . 0-05  inch 

2  .         . 0-09     " 

3 0-18     •* 

4 0-20     " 

5 0-25     " 

6 0-21     ** 

7 0-27     " 

8 0-30    ♦* 

9 0-21     '* 

10 0-20    ** 

11 0-16     " 

Comparing  the  three  lines  together,  it  will  be  observed 
that  the  velocity  diminishes  as  we  descend  the  glacier.     In 


ALPINE  SCULPTURE  263 

100  hours  the  maximum  motion  of  the  three  lines  respec- 
tive! j  is  as  follows; 

Maximum  Motion  in  100  hours 

Line  A 56  inches 

•*     B 45      *' 

"     C 30      " 

This  deportment  explains  an  appearance  which  must 
strike  every  observer  who  looks  upon  the  Morteratsch  from 
the  Piz  Languard,  or  from  the  new  Bernina  Koad.  A  me- 
dial moraine  runs  along  the  glacier,  commencing  as  a  nar- 
row streak,  but  toward  the  end  the  moraine  extending  in 
width,  until  finally  it  quite  covers  the  terminal  portion  of 
the  glacier.  The  cause  of  this  is  revealed  by  the  foregoing 
measurements,  which  prove  that  a  stone  on  the  moraine 
where  it  is  crossed  by  the  line  A  approaches  a  second  stone 
on  the  moraine  where  it  is  crossed  by  the  line  C  with  a 
velocity  of  twenty-six  inches  per  one  hundred  hours.  The 
moraine  is  in  a  state  of  longitudinal  compression.  Its  ma- 
terials are  more  and  more  squeezed  together,  and  they  must 
consequently  move  laterally  and  render  the  moraine  at  the 
terminal  portion  of  the  glacier  wider  than  above. 

The  motion  of  the  Morteratsch  glacier,  then,  diminishes 
as  we  descend.  The  maximum  motion  of  the  third  line  is 
thirty  inches  in  one  hundred  hours,  or  seven  inches  a  day 
— a  very  slow  motion ;  and  had  we  run  a  line  nearer  to  the 
end  of  the  glacier,  the  motion  would  have  been  slower  still. 
At  the  end  itself  it  is  nearly  insensible.*  Now,  I  submit  that 
this  is  not  the  place  to  seek  for  the  scooping  power  of  a 


*  The  snout  of  the  Aletsch  glacier  has  a  diurnal  motion  of  Jess  than  two 
inches,  while  a  mile  or  so  above  the  snout  the  velocity  is  eighteen  inches.  The 
spreading  out  of  the  moraine  is  here  very  striking. 


284  FRAGMENTS    OF  SCIENCE 

glacier.  The  opinion  appears  to  be  prevalent  tliat  it  is  tlie 
snout  of  a  glacier  that  must  act  the  part  of  plowshare;  and 
it  is  certainly  an  erroneous  opinion.  The  scooping  power 
will  exert  itself  most  where  the  weight  and  the  motion  are 
greatest.  A  glacier's  snout  often  rests  upon  matter  which 
has  been  scooped  from  the  glacier's  bed  higher  up.  I 
therefore  do  not  think  that  the  inspection  of  what  the  end 
of  a  glacier  does  or  does  not  accomplish  can  decide  this 
question. 

The  snout  of  a  glacier  is  potent  to  remove  anything 
against  which  it  can  fairly  abut;  and  this  power,  notwith- 
standing the  slowness  of  the  motion,  manifests  itself  at  the 
end  of  the  Morteratsch  glacier.  A  hillock,  bearing  pine- 
trees,  was  in  front  of  the  glacier  when  Mr.  Hirst  and  my- 
self inspected  its  end;  and  this  hillock  is  being  bodily 
removed  by  the  thrust  of  the  ice.  Several  of  the  trees  are 
overturned;  and  in  a  few  years,  if  the  glacier  continues 
its  reputed  advance,  the  mound  will  certainly  be  plowed 
away. 

The  question  of  Alpine  conformation  stands,  I  think, 
thus:  We  have,  in  the  first  place,  great  valleys,  such  as 
those  of  the  Ehine  and  the  Ehone,  which  we  might  con- 
veniently call  valleys  of  the  first  order.  The  mountains 
which  flank  these  main  valleys  are  also  cut  by  lateral  val- 
leys running  into  the  main  ones,  and  which  may  be  called 
valleys  of  the  second  order.  "When  these  latter  are  exam- 
ined, smaller  valleys  are  found  running  into  them,  which 
may  be  called  valleys  of  the  third  order.  Smaller  ravines 
and  depressions,  again,  join  the  latter,  which  may  be  called 
valleys  of  the  foarth  order,  and  so  on  until  we  reach  streaks 
and  cuttings  so  minute  as  not  to  merit  the  name  of  valleys 
at  all.     At  the  bottom  of  every  valley  we  have  a  stream, 


ALPINE   SCULPTURE  265 

diminishing  in  magnitude  as  the  order  of  the  valley  ascends, 
carving  the  earth  and  carrying  its  materials  to  lower  levels. 
We  find  that  the  larger  valleys  have  been  filled  for  untold 
ages  by  glaciers  of  enormous  dimensions,  always  moving, 
grinding  down  and  tearing  away  the  rocks  over  which  they 
passed.  We  have,  moreover,  on  the  plains  at  the  foot  of 
the  mountains,  and  in  enormous  quantities,  the  very  matter 
derived  from  the  sculpture  of  the  mountains  themselves. 

The  plains  of  Italy  and  Switzerland  are  cumbered  by 
the  debris  of  the  Alps.  The  lower,  wider,  and  more  level 
valleys  are  also  filled  to  unknown  depths  with  the  materials 
derived  from  the  higher  ones.  In  the  vast  quantities  of 
moraine- matter  which  cumber  many  even  of  the  higher  val- 
leys we  have  also  suggestions  as  to  the  magnitude  of  the 
erosion  which  has  taken  place.  This  moraine-matter,  more- 
over, can  only  in  small  part  have  been  derived  from  the 
falling  of  rocks  upon  the  ancient  glacier;  it  is  in  great  part 
derived  from  the  grinding  and  the  plowing-out  of  the  gla- 
cier itself.  This  accounts  for  the  magnitude  of  many  of  the 
ancient  moraines,  which  date  from  a  period  when  almost 
all  the  mountains  were  covered  with  ice  and  snow,  and 
when,  consequently,  the  quantity  of  moraine- matter  derived 
from  the  naked  crests  cannot  have  been  considerable. 

The  erosion  theory  ascribes  the  formation  of  Alpine 
valleys  to  the  agencies  here  briefly  referred  to.  It  in- 
vokes nothing  but  true  causes.  Its  artificers  are  still  there, 
though,  it  may  be,  in  diminished  strength ;  and,  if  they  are 
granted  sufficient  time,  it  is  demonstrable  that  they  are  com- 
petent to  produce  the  effects  ascribed  to  them.  And  what 
does  the  fracture  theory  offer  in  comparison?  From  no 
possible  application  of  this  theory,  pure  and  simple,  can 

we  obtain  the  slopes  and  forms  of  the  mountains.     Erosion 

Science — Y— 12 


266  FRAGMENTS    OF   SCIENCE 

must  in  the  long  run  be  invoked,  and  its  power  therefore 
conceded.  The  fracture  theory  infers  from  the  disturb- 
ances of  the  Alps  the  existence  of  fissures;  and  this  is  a 
probable  inference.  But  that  they  were  of  a  magnitude 
sufficient  to  produce  the  conformation  of  the  Alps,  and 
that  they  followed,  as  the  Alpine  valleys  do,  the  lines  of 
natural  drainage  of  the  country,  are  assumptions  which 
do  not  appear  to  me  to  be  justified  either  by  reason  or 
by  observation. 

There  is  a  grandeur  in  the  secular  integration  of  small 
effects  implied  by  the  theory  of  erosion  almost  superior  to 
that  involved  in  the  idea  of  a  cataclj^sm.  Think  of  the 
ages  which  must  have  been  consumed  in  the  execution  of 
this  colossal  sculpture.  The  question  may,  of  course,  be 
pushed  further.  Think  of  the  ages  which  the  molten  earth 
required  for  its  consolidation.  But  these  vaster  epochs 
lack  sublimity  through  our  inability  to  grasp  them.  They 
bewilder  us,  but  they  fail  to  make  a  solemn  impression. 
The  genesis  of  the  mountains  comes  more  within  the  sco-pe 
of  the  intellect,  and  the  majesty  of  the  operation  is  en- 
hanced by  our  partial  ability  to  conceive  it.  In  the  fall- 
ing of  a  rock  from  a  mountain-head,  in  the  shoot  of  an 
avalanche,  in  the  plunge  of  a  cataract,  we  often  see  more 
impressive  illustrations  of  the  power  of  gravity  than  in  the 
motions  of  the  stars.  When  the  intellect  has  to  intervene, 
and  calculation  is  necessary  to  the  building  up  of  the  con- 
ception, the  expansion  of  the  feelings  ceases  to  be  propor- 
tional to  the  magnitude  of  the  phenomena. 


I  will  here  record  a  few  other  measurements  executed 
on  the  Eosegg  glacier:  the  line  was  staked  out  across  the 


ALPINE  SCULPTURE  267 

trunk  formed  by  the  junction  of  the  Eosegg  proper  with 
the  Tschierva  glacier,  a  short  distance  below  the  rocky 
promontory  called  Agaliogs. 


Rose 
.  of  Stake 

gg  Glacier 

Hourly  Motion 

1 

.      0-01  inch 

2 

.      0-05     ' 

3 

.      0-07     ' 

4 

.      0-10     ' 

5 

.      Oil     ' 

6 

.      013     ' 

7 

.      0-U     * 

8 

.      0-18     ' 

9 

.      0-24     ♦ 

10 

.      0-23     ' 

11 

.      0-24     ' 

This  is  an  extremely  slowly  moving  glacier;  the  maxi- 
mum motion  hardly  amounts  to  seven  inches  a  day.  Cre- 
vasses prevented  us  from  continuing  the  line  quite  across 
the  glacier. 


RECENT   EXPERIMENTS   ON   FOG-SIGNALS* 

THE  care  of  its  sailors  is  one  of  the  first  duties  of  a 
maritime  people,  and  one  of  the  sailor's  greatest 
dangers  is  his  proximity  to  the  coast  at  night. 
Hence  the  idea  of  warning  him  of  such  proximity  by 
beacon -fires  placed  sometimes  on  natural  eminences  and 
sometimes  on  towers  built  expressly  for  the  purpose. 
Close  to  Dover  Castle,  for  example,  stands  an  ancient 
Pharos  of  this  description. 

As  our  marine  increased  greater  skill  was  invoked,  and 
lamps  reinforced  by  parabolic  reflectors  poured  their  light 
upon  the  sea.  Several  of  these  lamps  were  sometimes 
grouped  together  so  as  to  intensify  the  light,  which  at  a 
little  distance  appeared  as  if  it  emanated  from  a  single 
source.  This  *' catoptric'*  form  of  apparatus  is  still  to  some 
extent  employed  in  our  lighthouse -service,  but  for  a  long 
time  past  it  has  been  more  and  more  displaced  by  the  great 
lenses  devised  by  the  illustrious  Frenchman,  Fresnel. 

In  a  first-class  ** dioptric''  apparatus  the  light  emanates 
from  a  lamp  with  several  concentric  wicks,  the  flame  of 
which,  being  kindled  by  a  very  active  draught,  attains  to 
great  intensity.  In  fixed  lights  the  lenses  refract  the  rays 
issuing  from  the  lamp  so  as  to  cause  them  to  form  a  lumi- 
nous sheet  which  grazes  the  sea-horizon.      In  revolving 

*  A  disoourse  delivered  ia  the  Royal  Iustituti<Ki,  March  22,  1878. 

(268) 


RECENT  EXPERIMENTS    ON   FOG-SIGNALS  269 

lights  the  lenses  gather  up  the  rays  into  distinct  beams, 
resembling  the  spokes  of  a  wheel,  which  sweep  over  the 
sea  and  strike  the  eye  of  the  mariner  in  succession. 

It  is  not  for  clear  weather  that  the  greatest  strengthen- 
ing of  the  light  is  intended,  for  here  it  is  not  needed.  Nor 
is  it  for  densely  foggy  weather,  for  here  it  is  ineffectual. 
But  it  is  for  the  intermediate  stages  of  hazy,  snowy,  or 
rainy  weather,  in  which  a  powerful  light  can  assert  itself, 
while  a  feeble  one  is  extinguished.  The  usual  first-order 
lamp  is  one  of  four  wicks,  but  Mr.  Douglass,  the  able  and 
indefatigable  engineer  of  the  Trinity  House,  has  recently 
raised  the  number  of  the  wicks  to  six,  which  produce  a 
very  noble  flame.  To  Mr.  Wigham,  of  Dublin,  we  are  in- 
debted for  the  successful  application  of  gas  to  lighthouse 
illumination.  In  some  lighthouses  his  power  varies  from 
28  jets  to  108  jets,  while  in  the  lighthouse  of  Galley  Head 
three  burners  of  the  largest  size  can  be  employed,  the  max- 
imum number  of  jets  being  324.  These  larger  powers  are 
invoked  only  in  case  of  fog,  the  28-jet  burner  being  amply 
sufficient  for  clear  weather.  The  passage  from  the  small 
burner  to  the  large,  and  from  the  large  burner  to  the  small, 
is  made  with  ease,  rapidity,  and  certainty.  This  employ- 
ment of  gas  is  indigenous  to  Ireland,  and  the  Board  of 
Trade  has  exercised  a  wise  liberality  in  allowing  every 
facility  to  Mr.  Wigham  for  the  development  of  his  inven- 
tion. 

The  last  great  agent  employed  in  lighthouse  illumination 
is  electricity.  It  was  in  this  Institution,  beginning  in  1831, 
that  Faraday  proved  the  existence  and  illustrated  the  laws 
of  those  induced  currents  which  in  our  day  have  received 
such  astounding  development.  In  relation  to  this  subject 
Faraday's  words  have  a  prophetic  ring.     " I  have  rather, " 


270  FRAGMENTS    OF  SCIENCE 

he  writes,  in  1831,  "been  desirous  of  discovering  new  tacts 
and  new  relations  dependent  on  magneto- electric  induction 
tlian  of  exalting  the  force  of  those  already  obtained,  being 
assured  that  the  latter  would  find  their  full  development 
hereafter."  The  labors  of  Holmes,  of  the  Paris  Alliance 
Company,  of  Wilde,  and  of  Gramme,  constitute  a  brilliant 
b.lfilment  of  this  prediction. 

But,  as  regards  the  augmentation  of  power,  the  greatest 
itep  hitherto  made  was  independently  taken  a  few  years 
ago  by  Dr.  "Werner  Siemens  and  Sir  Charles  Wheatstone. 
Through  the  application  of  their  discovery  a  machine  en- 
dowed with  an  infinitesimal  charge  of  magnetism  may,  by 
a  process  of  accumulation  at  compound  interest,  be  caused 
so  to  enrich  itself  magnetically  as  to  cast  by  its  perform- 
ance all  the  older  machines  into  the  shade.  The  light  now 
before  you  is  that  of  a  small  machine  placed  downstairs, 
and  worked  there  by  a  minute  steam-engine.  It  is  a  light 
of  about  1,000  candles;  and  for  it,  and  for  the  steam-engine 
that  works  it,  our  members  are  indebted  to  the  liberality  of 
Dr.  William  Siemens,  who  in  the  most  generous  manner 
has  presented  the  machine  to  this  Institution.  After  an 
exhaustive  trial  at  the  South  Foreland,  machines  on  the 
principle  of  Siemens,  but  of  far  greater  power  than  this 
one,  have  been  recently  chosen  by  the  Elder  Brethren  of 
the  Trinity  House  for  the  two  lighthouses  at  the  Lizard 
Point. 

Our  most  intense  lights,  including  the  six- wick  lamp, 
the  Wigham  gas  light,  and  the  electric  light,  being  intended 
to  aid  the  mariner  in  heavy  weather,  may  be  regarded,  in 
a  certain  sense,  as  fog- signals.  But  fog,  when  thick,  is 
intractable  to  light.  The  sun  cannot  penetrate  it,  much  less 
any  terrestrial  source  of  illumination.     Hence  the  necessity 


RECENT   EXPERIMENTS    ON  FOQ-SIGNALS  271 

of  employing  sound-signals  in  dense  fogs.  Bells,  gongs, 
horns,  whistles,  guns,  and  syrens  have  been  used  for  this 
purpose;  but  it  is  mainly,  if  not  wholly,  with  explosive 
signals  that  we  have  now  to  deal.  The  gun  has  been  em- 
ployed with  useful  effect  at  the  North  Stack,  near  Holy- 
head, on  the  Kish  Bank  near  Dublin,  at  Lundy  Island, 
and  at  other  points  on  our  coasts.  During  the  long,  labo- 
rious, and  I  venture  to  think  memorable,  series  of  observa- 
tions conducted  under  the  auspices  of  the  Elder  Brethren 
of  the  Trinity  House  at  the  South  Foreland  in  1872  and 
1873,  it  was  proved  that  a  short  6>^-inch  howitzer,  firing 
3  lbs.  of  powder,  yielded  a  louder  report  than  a  long 
18-pounder  firing  the  same  charge.  Here  was  a  hint  to  be 
acted  on  by  the  Elder  Brethren.  The  effectiveness  of  the 
sound  depended  on  the  shape  of  the  gun,  and  as  it  could 
not  be  assumed  that  in  the  howitzer  we  had  hit  accident- 
ally upon  the  best  possible  shape,  arrangements  were  made 
with  the  War  Office  for  the  construction  of  a  gun  specially 
calculated  to  produce  the  loudest  sound  attainable  from 
the  combustion,  of  3  lbs.  of  powder.  To  prevent  the  un- 
necessary landward  waste  of  the  sound,  the  gun  was  fur- 
nished with  a  parabolic  muzzle,  intended  to  project  the 
sound  over  the  sea,  where  it  was  most  needed.  The  con- 
struction of  this  gun  was  based  on  a  searching  series  of 
experiments  executed  at  Woolwich  with  small  models,  pro- 
vided with  muzzles  of  various  kinds.  A  drawing  of  the 
gun  is  annexed  (p.  272).  It  was  constructed  on  the  prin- 
ciple of  the  revolver,  its  various  chambers  being  loaded 
and  brought  in  rapid  succession  into  the  firing  position. 
The  performance  of  the  gun  proved  the  correctness  of  the 
principles  on  which  its  construction  was  based. 

An  incidental  point  of   some   interest  was  decided  by 


272 


FRAGMENTS   OF  SCIENCE 


the  earliest  Woolwich  experiments.  It  Lad  been  a  widely 
spread  opinion  among  artillerists,  that  a  bronze  gun  pro- 
duces a  specially  loud  report.  I  doubted  from  the  outset 
whether  this  would  help  us;  and,  in  a  letter  dated  22d 
April,  1874,  I  ventured  to  express  myself  thus:  "The 
report  of  a  gun,  as  affecting  an  observer  close  at  hand,  is 
made  up  of  two  factors — ^the  sound  due  to  the  shock  of  the 
air  by  the  violently  expanding  gas,  and  the  sound  derived 
from  the   vibrations   of  the  gun,   which,   to  some   extent, 


Fig.  6.— Breech-loading  Fog-signal  Gun,  with  Bell  Mouth/  proposed  by 
Major  Maitland,  R.  A.,  Assistant  Superintendent. 

rings  like  a  bell.  This  latter,  I  apprehend,  will  disappear 
at  considerable  distances."  The  result  of  subsequent  trial, 
as  reported  by  General  Campbell,  is,  "that  the  sonorous 
qualities  of  bronze  are  greatly  superior  to  those  of  cast-iron 
at  short  distances,  but  that  the  advantage  lies  with  the  baser 
metal  at  long  ranges.  '* ' 


*  The  carriage  of  this  gun  has  been  modified  in  construction  since  this 
drawing  was  made. 

*  General  Campbell  assigns  a  true  cause  for  this  difference.    The  ring  of  the 


RECENT   EXPERIMENTS    ON    FOG-SIGNALS  273 

Coincident  with  these  trials  of  guns  at  Woolwich,  gun- 
cotton  was  thought  of  as  a  probably  effective  sound-pro- 
ducer. From  the  first,  indeed,  theoretic  considerations 
caused  me  to  fix  my  attention  persistently  on  this  sub- 
stance; for  the  remarkable  experiments  of  Mr.  Abel, 
whereby  its  rapidity  of  combustion  and  violently  explo- 
sive energy  are  demonstrated,  seemed  to  single  it  out  as 
a  substance  eminently  calculated  to  fulfil  the  conditions 
necessary  to  the  production  of  an  intense  wave  of  sound. 
What  those  conditions  are  we  shall  now  more  particularly 
inquire,  calling  to  our  aid  a  brief  but  very  remarkable 
paper,  published  by  Professor  Stokes  in  the  "Philosophical 
Magazine"  for  1868. 

The  explosive  force  of  gunpowder  is  known  to  depend 
on  the  sudden  conversion  of  a  solid  body  into  an  intensely 
heated  gas.  Now  the  work  which  the  artillerist  requires 
the  expanding  gas  to  perform  is  the  displacement  of  the 
projectile,  besides  which  it  has  to  displace  the  air  in  front 
of  the  projectile,  which  is  backed  by  the  whole  pressure  of 
the  atmosphere.  Such,  however,  is  not  the  work  that  we 
want  our  gunpowder  to  perform.  We  wish  to  transmute 
its  energy  not  into  the  mere  mechanical  translation  of  either 
shot  or  air,  but  into  vibratory  motion.  We  want  pulses  to 
be  formed  which  shall  propagate  themselves  to  vast  dis- 
tances through  the  atmosphere,  and  this  requires  a  certain 
choice  and  management  of  the  explosive  material. 

A  sound-wave  consists  essentially  of  two  parts — a  con- 
densation and  a  rarefaction.  Now,  air  is  a  very  mobile 
fluid,  and  if  the  shock  imparted  to  it  lack  due  promptness, 

bronze  gun  represents  so  much  energy  withdrawn  from  the  explosive  force  of 
the  gunpowder.  Further  experiments  would,  however,  be  needed  to  place  the 
superiority  of  the  cast-iron  gun  at  a  distance  beyond  question. 


274  FRAGMENTS   OF  SCIENCE 

the  wave  is  not  produced.  Consider  the  case  of  a  common 
clock  pendulum,  which  oscillates  to  and  fro,  and  which 
might  be  expected  to  generate  corresponding  pulses  in  the 
air.  When,  for  example,  the  bob  moves  to  the  right,  the 
air  to  the  right  of  it  might  be  supposed  to  be  condensed, 
while  a  partial  vacuum  might  be  supposed  to  follow  the 
bob.  As  a  matter  of  fact,  we  have  nothing  of  the  kind. 
The  air  particles  in  front  of  the  bob  retreat  so  rapidly,  and 
those  behind  it  close  so  rapidly  in,  that  no  sound-pulse  is 
formed.  The  mobility  of  hydrogen,  moreover,  being  far 
greater  than  that  of  air,  a  prompter  action  is  essential  to 
the  formation  of  sonorous  waves  in  hydrogen  than  in  air. 
It  is  to  this  rapid  power  of  readjustment,  this  refusal,  so 
to  speak,  to  allow  its  atoms  to  be  crowded  together  or  to 
be  drawn  apart,  that  Professor  Stokes,  with  admirable  pen- 
etration, refers  the  damping  power,  first  described  by  Sir 
John  Leslie,  of  hydrogen  upon  sound. 

A  tuning-fork  which  executes  256  complete  vibrations 
in  a  second,  if  struck  gently  on  a  pad  and  held  in  free  air, 
emits  a  scarcely  audible  note.  It  behaves  to  some  extent 
like  the  pendulum  bob  just  referred  to.  This  feebleness  is 
due  to  the  prompt  "reciprocating  flow"  of  the  air  between 
the  incipient  condensations  and  rarefactions,  whereby  the 
formation  of  sound-pulses  is  forestalled.  Stokes,  however, 
has  taught  us  that  this  flow  may  be  intercepted  by  placing 
the  edge  of  a  card  in  close  proximity  to  one  of  the  corners 
of  the  fork.  An  immediate  augmentation  of  the  sound  of 
the  fork  is  the  consequence. 

The  more  rapid  the  shock  imparted  to  the  air,  the 
greater  is  the  fractional  part  of  the  energy  of  the  shock 
converted  into  wave-motion.  And  as  different  kinds  of 
gunpowder  vary  considerably  in  their  rapidity  of  combus- 


RECENT  EXPERIMENTS   ON  FOG-SIGNALS  275 

tion,  it  may  be  expected  that  they  will  also  vary  as  pro- 
ducers of  sound.  This  theoretic  inference  is  completely 
verified  by  experiment.  In  a  series  of  preliminary  trials 
conducted  at  Woolwich  on  the  4tli  of  June,  1876,  the 
sound-producing  powers  of  four  different  kinds  of  powder 
were  determined.  In  the  order  of  the  size  of  their  grains 
they  bear  the  names  respectively  of  Fine-grain  (F.  G.), 
Large-grain  (L.  Gr.),  Rifle  Large-grain  (R.  L.  G.),  and 
Pebble-grain  (P.)  (See  annexed  figures.)  The  charge  in 
each    case    amounted   to   43^   lbs.;    four    24-lb.    howitzers 


F.G.  L.G.  R.L.Q.  P. 

Fig.  7. 

being  employed  to  fire  the  respective  charges.  There 
were  eleven  observers,  all  of  whom,  without  a  single  dis- 
sentient, pronounced  the  sound  of  the  fine -grain  powder 
loudest  of  all.  In  the  opinion  of  seven  of  the  eleven  the 
large-grain  powder  came  next;  seven  also  of  the  eleven 
placed  the  rifle  large-grain  third  on  the  list;  while  they 
were  again  unanimous  in  pronouncing  the  pebble-powder 
the  worst  sound -producer.  These  differences  are  entirely 
due  to  differences  in  the  rapidity  of  combustion.  All  who 
have  witnessed  the  performance  of  the  80-ton  gun  must 
have  been  surprised  at  the  mildness  of  its  thunder.  To 
avoid  the  strain  resulting  from  quick  combustion,  the 
powder  employed  is  composed  of  lumps  far  larger  than 
those  of  the  pebble-powder  above  referred  to.     In  the  long 


276  FRAGMENTS   OF  SCIENCE 

tube  of  the  gun  these  lumps  of  solid  matter  gradually  re- 
solve themselves  into  gas,  which  on  issuing  from  the  muz- 
zle imparts  a  kind  of  push  to  the  air,  instead  of  the  sharp 
shock  necessary  to  form  the  condensation  of  an  intensely 
sonorous  wave. 

These  are  some  of  the  physical  reasons  why  gun-cotton 
might  be  regarded  as  a  promising  fog-signal.  Firing  it  as 
we  have  been  taught  to  do  by  Mr.  Abel,  its  explosion  is 
more  rapid  than  that  of  gunpowder.  In  its  case  the  air 
particles,  alert  as  they  are,  will  not,  it  might  be  presumed, 
be  able  to  slip  from  condensation  to  rarefaction  with  a  ra- 
pidity sufficient  to  forestall  the  formation  of  the  wave.  On 
d  priori  grounds,  then,  we  are  entitled  to  infer  the  effective- 
ness of  gun-cotton,  while  in  a  great  number  of  comparative 
experiments,  stretching  from  1874  to  the  present  time,  this 
inference  has  been  verified  in  the  most  conclusive  manner. 

As  regards  explosive  material,  and  zealous  and  accom- 
plished help  in  the  use  of  it,  the  resources  of  Woolwich 
Arsenal  have  been  freely  placed  at  the  disposal  of  the 
Elder  Brethren.  General  Campbell,  General  Younghus- 
band.  Colonel  Fraser,  Colonel  Maitland,  and  other  officers, 
have  taken  an  active  personal  part  in  the  investigation, 
and  in  most  cases  have  incurred  the  labor  of  reducing  and 
reporting  on  the  observations.  Guns  of  various  forms  and 
sizes  have  been  invoked  for  gunpowder,  while  gun-cotton 
has  been  fired  in  free  air  and  in  the  foci  of  parabolic 
reflectors. 

On  the  22d  of  February,  1875,  a  number  of  small  guns, 
cast  specially  for  the  purpose — some  with  plain,  some  with 
conical,  and  some  with  parabolic  muzzles — firing  4  oz.  of 
fine-grain  powder,  were  pitted  against  4  oz.  of  gun-cotton 
detonated  both  in  the  open,  and  in  the  focus  of  a  parabolic 


RECENT  EXPERIMENTS   ON  FOG-SIGNALS  277 

reflector.'  The  sound  produced  by  the  gun-cotton,  rein- 
forced by  the  reflector,  was  unanimously  pronounced  loud- 
est of  all.  With  equal  unanimity,  the  gun-cotton  deto- 
nated in  free  air  was  placed  second  in  intensity.  Though 
the  same  charge  was  used  throughout,  the  guns  differed 
notably  among  themselves,  but  none  of  them  came  up  to 
the  gun-cotton,  either  with  or  without  the  reflector.  A 
second  series,  observed  from  a  different  distance  on  the 
same  day,  confirmed  to  the  letter  the  foregoing  result. 

As  a  practical  point,  however,  the  comparative  cost  of 
gun-cotton  and  gunpowder  has  to  be  taken  into  account, 
though  considerations  of  cost  ought  not  to  be  stretched 
too  far  in  cases  involving  the  safety  of  human  life.  In 
the  earlier  experiments,  where  quantities  of  equal  price 
were  pitted  against  each  other,  the  results  were  somewhat 
fluctuating.  Indeed,  the  perfect  manipulation  of  the  gun- 
cotton  required  some  preliminary  discipline — ^promptness, 
certainty,  and  effectiveness  of  firing,  augmenting  as  expe- 
rience increased.  As  1  lb.  of  gun-cotton  costs  as  much  as 
8  lbs.  of  gunpowder,  these  quantities  were  compared  to- 
gether on  the  22d  of  February.  The  guns  employed  to 
discharge  the  gunpowder  were  a  12 -lb.  brass  howitzer,  a 
24-lb.  cast-iron  howitzer,  and  the  long  18-pounder  em- 
Toloyed  at  the  South  Foreland.  The  result  was,  that  the 
24-lb.  howitzer,  firing  3  lbs.  of  gunpowder,  had  a  slight 
advantage  over  1  lb.  of  gun-cotton  detonated  in  the  open; 
while  the  12-lb.  howitzer  and  the  18-pounder  were  both 
beaten  by  the  gun-cotton.  On  the  2d  of  May,  on  the  other 
hand,  the  gun-cotton  is  reported  as  having  been  beaten 
by  all  the  guns. 

*  For  charges  of  this   weight  the  reflector  is  of  moderate  size,  and  may  be 
employed  without  fear  of  fracture. 


278  FRAGMENTS   OF  SCIENCE 

Meanwhile,  the  parabolic-muzzle  gun,  expressly  in 
tended  for  fog-signalling,  was  pushed  rapidly  forward, 
and  on  March  22  and  23,  1876,  its  power  was  tested  at 
Shoeburyness.  Pitted  against  it  were  a  16-pounder,  a 
5J^-inch  howitzer,  Ij^  lb.  of  gun-cotton  detonated  in  the 
focus  of  a  reflector  (see  annexed  figure),  and  1^  lb.  of 
gun-cotton  detonated  in  free  air.     On  this  occasion  nine- 


Fio.  8.— Gun-cotton  Slab  (1^  lb.)  Detonated  in  the  Focus  of  a  Cast-iron  Reflector 

teen  different  series  of  experiments  were  made,  when  the 
new  experimental  gun,  firing  a  3-lb.  charge,  demonstrated 
its  superiority  over  all  guns  previously  employed  to  fire 
the  same  charge.  As  regards  the  comparative  merits  of 
the  gun-cotton  fired  in  the  open,  and  the  gunpowder  fired 
from  the  new  gun,  the  mean  values  of  their  sounds  were 
the   same.      Fired  in  the   focus  of  the  reflector,  the  gun- 


RECENT  EXPERIMENTS   ON  FOG-SIGNALS  279 

cotton  clearly  dominated  over  all  the  other  sound-pro- 
ducers. * 

The  whole  of  the  observations  here  referred  to  were 
embraced  by  an  angle  of  about  70°,  of  which  60°  lay  oa 
the  one  side  and  20°  on  the  other  side  of  the  line  of  fire. 
The  shots  were  heard  by  eleven  observers  on  board  the 
** Galatea,'*  which  took  up  positions  varying  from  2  miles 
to  133^  miles  from  the  firing-point.  In  all  these  observa- 
tions, the  reinforcing  action  of  the  reflector,  and  of  the 
parabolic  muzzle  of  the  gun,  came  into  play.  But  the  re- 
inforcement of  the  sound  in  one  direction  implies  its  with- 
drawal from  some  other  direction,  and  accordingly  it  was 
found  that  at  a  distance  of  5J  miles  from  the  firing-point, 
and  on  a  line  including  nearly  an  angle  of  90°  with  th« 
line  of  fire,  the  gun-cotton  in  the  open  beat  the  new  gun; 
while  behind  the  station,  at  distances  of  83^  miles  and  ISJ^ 
miles  respectively,  the  gun-cotton  in  the  open  beat  both 
the  gun  and  the  gun-cotton  in  the  reflector.  This  result 
is  rendered  more  important  by  the  fact  that  the  sound 
reached  the  Mucking  Light,  a  distance  of  183^  miles, 
against  a  light  wind  which  was  blowing  at  the  time. 

Most,  if  not  all,  of  our  ordinary  sound-producers  send 
forth  waves  which  are  not  of  uniform  intensity  throughout. 
A  trumpet  is  loudest  in  the  direction  of  its  axis.  The 
same  is  true  of  a  gun.  A  bell,  with  its  mouth  pointed 
upward  or  downward,  sends  forth  waves  which  are  far 
denser  in  the  horizontal  plane  passing  through  the  bell 
than  at  an  angular  distance  of  90°  from  that  plane.  The 
oldest  bellhangers  must  have  been  aware  of  the  fact  tbat 


>  The  reflector  was  fractured  by  the  explosion,  but  it  did  good  service 
afterward. 


280  FRAGMENTS   OF  SCIENCIS 

the  sides  of  the  bell,  and  not  it^  mouth,  emitted  the  strong- 
est sound,  their  practice  being  probably  determined  by  this 
knowledge.  Our  slabs  of  gun-cotton  also  emit  waves  of 
difEerent  densities  in  different  parts.  It  has  occurred  in 
the  experiments  at  Shoeburyness  that  when  the  broad  side 
of  a  slab  was  turned  toward  the  suspending  wire  of  a  sec- 
ond slab  six  feet  distant,  the  wire  was  cut  by  the  explo- 
sion, while  when  the  edge  of  the  slab  was  turned  to  the 
wire  this  never  occurred.  To  the  circumstance  that  the 
broad  sides  of  the  slabs  faced  the  sea  is  probably  to  be 
ascribed  the  remarkable  fact  observed  on  March  23,  that 
in  two  directions,  not  far  removed  from  the  line  of  fire,  the 
gun-cotton  'detonated  in  the  open  had  a  slight  advantage 
over  the  new  gun. 

Theoretic  considerations  rendered  it  probable  that  the 
shape  and  size  of  the  exploding  mass  would  affect  the  con- 
stitution of  the  wave  of  sound.  I  did  not  think  large  rect- 
angular slabs  the  most  favorable  shape,  and  accordingly 
proposed  cutting  a  large  slab  into  fragments  of  different 
sizes,  and  pitting  them  against  each  other.  The  differences 
between  the  sounds  were  by  no  means  so  great  as  the  dif- 
ferences in  the  quantities  of  explosive  material  might  lead 
one  to  expect.  The  mean  values  of  eighteen  series  of  ob- 
servations made  on  board  the  *' Galatea,"  at  distances  vary- 
ing from  1}  mile  to  4*8  miles,  were  as  follows: 


"Weights     .         .         .        4  oz.         6  oz.         9  oz.         12  oz. 
Value  of  sound .         .         3-12  3-34  4-0  4-03 


These  charges  were  cut  from  a  slab  of  dry  gun-cotton 
about  If  inch  thick:  they  were  squares  and  rectangles  of 
the  following   dimensions:   4  oz.,    2    inches   by  2   inches; 


RECENT  EXPERIMENTS   ON  FOG-SIGNALS  281 

6  oz.,  2  inclies  bj  3  inclies;  9  oz.,  8  inches  by  3  inches; 
12  oz.,  2  inches  by  6  inches. 

The  numbers  under  the  respective  weights  express  the 
recorded  value  of  the  sounds.  They  must  be  simply  taken 
as  a  ready  means  of  expressing  the  approximate  relative 
intensity  of  the  sounds  as  estimated  by  the  ear.  When  we 
find  a  9-oz.  charge  marked  4,  and  a  12-oz.  charge  marked 
4*03,  the  two  sounds  may  be  regarded  as  practically  equal 
in  intensity,  thus  proving  that  an  addition  of  30  per  cent 
In  the  larger  charges  produces  no  sensible  difference  in 
the  sound.  Were  the  sounds  estimated  by  some  physical 
means,  instead  of  by  the  ear,  the  values  of  the  sounds  at 
the  distances  recorded  would  not,  in  my  opinion,  show  a 
greater  advance  with  the  increase  of  material  than  that 
indicated  by  the  foregoing  numbers.  Subsequent  experi- 
ments rendered  still  more  certain  the  effectiveness,  as  well 
as  the  economy,  of  the  smaller  charges  of  gun-cotton. 

It  is  an  obvious  corollary  from  the  foregoing  experi- 
ments that  on  our  **nesses'*  and  promontories,  where  the 
land  is  clasped  on  both  sides  for  a  considerable  distance 
by  the  sea — where,  therefore,  the  sound  has  to  propagate 
itself  rearward  as  well  as  forward — ^the  use  of  the  parabolic 
gun,  or  of  the  parabolic  reflector,  might  be  a  disadvantage 
rather  than  an  advantage.  Here  gun-cotton,  exploded  in 
the  open,  forms  the  most  appropriate  source  of  sound. 
This  remark  is  especially  applicable  to  such  lightships  as 
are  intended  to  spread  the  sound  all  round  them  as  from 
central  foci.  As  a  signal  in  rock  lighthouses,  where  nei- 
ther syren,  steam- whistle,  nor  gun  could  be  mounted;  and 
as  a  handy  fleet- signal,  dispensing  with  the  lumber  of  spe- 
cial signal-guns,  the  gun-cotton  will  prove  invaluable. 
But  in  most  of  these  cases  we  have  the  drawback  that 


282  FRAGMENTS   OF  SCIENCE 

local  damage  may  be  done  by  the  explosion.  The  lantern 
of  the  rock  lighthouse  might  suffer  from  concussion  near 
at  hand,  and  though  mechanical  arrangements  might  be 
devised,  both  in  the  case  of  the  lighthouse  and  of  the 
ship's  deck,  to  place  the  firing-point  of  the  gun-cotton  at 
a  safe  distance,  no  such  arrangement  could  compete,  as 
regards  simplicity  and  effectiveness,  with  the  expedient 
of  a  gun-cotton  rocket.  Had  such  a  means  of  signalling 
existed  at  the  Bishop's  Rock  Lighthouse,  the  ill-fated 
"Schiller"  might  have  been  warned  of  her  approach  to 
danger  ten,  or  it  may  be  twenty,  miles  before  she  reached 
the  rock  which  wrecked  her.  Had  the  fleet  possessed  such 
a  signal,  instead  of  the  ubiquitous  but  ineffectual  whistle, 
the  *' Iron  Duke"  and  "Vanguard"  need  never  have  come 
into  collision. 

It  was  the  necessity  of  providing  a  suitable  signal  for 
rock  lighthouses,  and  of  clearing  obstacles  which  cast  an 
acoustic  shadow,  that  suggested  the  idea  of  the  gun-cotton 
rocket  to  Sir  Richard  Collinson,  Deputy  Master  of  the 
Trinity  House.  His  idea  was  to  place  a  disk  or  short 
cylinder  of  gun-cotton  in  the  head  of  a  rocket,  the  ascen- 
sional force  of  which  should  be  employed  to  carry  the 
disk  to  an  elevation  of  1,000  feet  or  thereabout,  where, 
by  the  ignition  of  a  fuse  associated  with  a  detonator,  the 
gun-cotton  should  be  fired,  sending  its  sound  in  all  direc- 
tions vertically  and  obliquely  down  upon  earth  and  sea. 
The  first  attempt  to  realize  this  idea  was  made  on  July 
18,  1876,  at  the  firework  manufactory  of  the  Messrs.  Brock, 
at  Nunhead.  Eight  rockets  were  then  fired,  four  being 
charged  with  5  oz.  and  four  with  7^  oz.  of  gun-cotton. 
They  ascended  to  a  great  height,  and  exploded  with  a 
very  loud  report  in  the  air.     On  July  27,  the  rockets  were 


RECENT  EXPERIMENTS   ON  FOG-SIGNALS  283 

tried  at  Shoeburjness.  The  most  noteworthy  result  on 
this  occasion  was  the  hearing  of  the  sounds  at  the  Mouse 
Lighthouse,  S^  miles  E.  by  S.,  and  at  the  Chapman  Light- 
house, Si/i  miles  W.  by  N. ;  that  is  to  say,  at  opposite 
sides  of  the  firing-point.  It  is  worthy  of  remark  that,  in 
the  case  of  the  Chapman  Lighthouse,  land  and  trees  inter- 
vened between  the  firing-point  and  the  place  of  observa- 
tion. "This,"  as  General  Younghusband  justly  remarked 
at  the  time,  "may  prove  to  be  a  valuable  consideration  if 
it  should  be  found  necessary  to  place  a  signal  station  in 
a  position  whence  the  sea  could  not  be  freely  observed." 
Indeed,  the  clearing  of  such  obstacles  was  one  of  the 
objects  which  the  inventor  of  the  rocket  had  in  view. 

With  reference  to  the  action  of  the  wind,  it  was 
thought  desirable  to  compare  the  range  of  explosions  pro- 
duced near  the  surface  of  the  earth  with  others  produced 
at  the  elevation  attainable  by  the  gun-cotton  rockets. 
Wind  and  weather,  however,  are  not  at  our  command; 
and  hence  one  of  the  objects  of  a  series  of  experiments 
conducted  on  December  13,  1876,  was  not  fulfilled.  It  is 
worthy,  however,  of  note  that  on  this  day,  with  smooth 
water  and  a  calm  atmosphere,  the  rockets  were  distinctly 
heard  at  a  distance  of  11*2  miles  from  the  firing -point. 
The  quantity  of  gun-cotton  employed  was  73^  oz.  On 
Thursday,  March  8,  1877,  these  comparative  experiments 
of  firing  at  high  and  low  elevations  were  pushed  still 
further.  The  gun-cotton  near  the  ground  consisted  of 
J^-lb.  disks,  suspended  from  a  horizontal  iron  bar  about 
43^  feet  above  the  ground.  The  rockets  carried  the  same 
quantity  of  gun-cotton  in  their  heads,  and  the  height  to 
which  they  attained,  as  determined  by  a  theodolite,  was 
from  800  to  900  feet.     The  day  was  cold,  with  occasional 


284  FRAGMENTS   OF  SCIENCE 

squalls  of  snow  and  hail,  the  direction  of  the  sound  being 
at  right  angles  to  that  of  the  wind.  Five  series  of  obser- 
vations were  made  on  board  the  "Vestal/'  at  distances 
varying  from  3  to  6  miles.  The  mean  value  of  the  explo- 
sions in  the  air  exceeded  that  of  the  explosions  near  the 
ground  by  a  small  but  sensible  quantity.  At  Windmill 
Hill,  Gravesend,  however,  which  was  nearly  to  leeward, 
and  6]/2  niiles  from  the  firing-point,  in  nineteen  cases  out 
of  twenty-four  the  disk  fired  near  the  ground  was  loud- 
est; while  in  the  remaining  five  the  rocket  had  the 
advantage. 

Toward  the  close  of  the  day  the  atmosphere  became 
very  serene.  A  few  distant  cumuli  sailed  near  the  hori- 
zon, but  the  zenith  and  a  vast  angular  space  all  round  it 
were  absolutely  free  from  cloud.  From  the  deck  of  the 
*' Galatea"  a  rocket  was  discharged,  which  reached  a  great 
elevation,  and  exploded  with  a  loud  report.  Following 
this  solid  nucleus  of  sound  was  a  continuous  train  of 
echoes,  which  retreated  to  a  continually  greater  distance, 
dying  gradually  off  into  silence  after  seven  seconds'  dura- 
tion. These  echoes  were  of  the  same  character  as  those 
so  frequently  noticed  at  the  South  Foreland  in  1872-73, 
and  called  by  me  ** aerial  echoes." 

On  the  23d  of  March  the  experiments  were  resumed, 
the  most  noteworthy  results  of  that  day's  observations 
being  that  the  sounds  were  heard  at  Tillingham,  10  miles 
to  the  IST.E.;  at  West  Mersea,  15i  miles  to  the  N.E.  by 
E. ;  at  Brightlingsea,  17J^  miles  to  the  N.B.;  and  at  Clac- 
ton  Wash,  20J^  miles  to  the  N.E.  by  H  E.  The  wind 
was  blowing  at  the  time  from  the  S.B.  Some  of  these 
sounds  were  produced  by  rockets,  some  by  a  24-lb,  how- 
itzer, and  some  by  an  8-inch  Maroon. 


RECENT  EXPERIMENTS   ON  FOG-SIGNALS  285 

In  December,  1876,  Mr.  Gardiner,  the  managing  director 
of  the  Cotton -powder  Company,  had  proposed  a  trial  of 
this  material  against  the  gun-cotton.  The  density  of  the 
cotton  he  urged  was  only  1-03,  while  that  of  the  powder 
was  1-70.  A  greater  quantity  of  explosive  material  being 
thus  compressed  into  the  same  volume,  Mr.  Gardiner 
thought  that  a  greater  sonorous  effect  must  be  produced 
by  the  powder.  At  the  instance  of  Mr.  Mackie,  who  had 
previously  gone  very  thoroughly  into  the  subject,  a  com- 
mittee of  the  Elder  Brethren  visited  the  cotton-powder 
manufactory,  on  the  banks  of  the  Swale,  near  Faversham, 
on  the  16th  of  June,  1877.  The  weights  of  cotton-powder 
employed  were  2  oz.,  8  oz.,  1  lb.,  and  2  lbs.,  in  the  form 
of  rockets  and  of  signals  fired  a  few  feet  above  the 
ground.  The  experiments  throughout  were  arranged  and 
conducted  by  Mr.  Mackie.  Our  desire  on  this  occasion 
was  to  get  as  near  to  windward  as  possible,  but  the  Swale 
and  other  obstacles  limited  our  distance  to  V/^  mile.  We 
stood  here  E.S.E.  from  the  firing-point  while  the  wind 
blew  fresh  from  the  N.E. 

The  cotton-powder  yielded  a  very  effective  report. 
The  rockets  in  general  had  a  slight  advantage  over  the 
same  quantities  of  material  fired  near  the  ground.  The 
loudness  of  the  sound  was  by  no  means  proportional  to 
the  quantity  of  the  material  exploded,  8  oz.  yielding  very 
nearly  as  loud  a  report  as  1  lb.  The  *' aerial  echoes,'* 
which  invariably  followed  the  explosion  of  the  rockets, 
were  loud  and  long-continued. 

On  the  17th  of  October,  1877,  another  series  of  experi- 
ments with  howitzers  and  rockets  was  carried  out  at 
Shoeburyness.  The  charge  of  the  howitzer  was  3  lbs. 
of  L.  G.  powder.     The  charges  of  the  rockets  were  12  oz., 


286  FRAGMENTS   OF  SCIENCE 

8  oz.,  4  oz.,  and  2  oz.  of  gun-cotton  respectively.  The 
gun  and  the  four  rockets  constituted  a  series,  and  eight 
series  were  fired  during  the  afternoon  of  the  17th.  The 
observations  were  made  from  the  ** Vestal"  and  the  *' Gal- 
atea," positions  being  successively  assumed  which  per- 
mitted the  sound  to  reach  the  observers  with  the  wind, 
against  the  wind,  and  across  the  wind.  The  distance  of 
the  *' Galatea"  varied  from  8  to  7  miles,  that  of  the 
** Vestal,"  which  was  more  restricted  in  her  movements, 
being  2  to  3  miles.  Briefly  summed  up,  the  result  is  that 
the  howitzer,  firing  a  3-lb.  charge,  which,  it  will  be  re- 
membered, was  our  best  gun  at  the  South  Foreland,  was 
beaten  by  the  12-oz.  rocket,  by  the  8-oz.  rocket,  and  by 
the  4-oz.  rocket.  The  S-oz.  rocket  alone  fell  behind  the 
howitzer. 

It  is  worth  while  recording  the  distances  at  which  some 
of  the  sounds  were  heard  on  the  day  now  referred  to: 


1.  Leigh    . 

.       6i  miles  W.KW. 

24  out  of  40  sounds  hesid 

2.  Girdler  Light-vessel        .     12       *'      S.E.  by  E. 

5 

3.  Reculvers 

.     Hi     "      S.E.  by  S. 

18 

4.  St.  Nicholas  . 

.     20       "      S.E. 

3 

5.   EppleBay      . 

.     22       '*      S.E.  by  E. 

19 

6.  Westgate 

.     23       '♦      S.E.  by  E. 

9 

*    *( 

7.  Kingsgate 

.     25       ♦'      S.E.  by  E. 

8 

The  day  was  cloudy,  with  occasional  showers  of  driz- 
zling rain;  the  wind  about  N.W.  by  N.  all  day;  at  times 
squally,  rising  to  a  force  of  6  or  7,  and  sometimes  drop- 
ping to  a  force  of  2  or  8.  The  station  at  Leigh  excepted, 
all  these  places  were  to  leeward  of  Shoeburyness.  At 
four  other  stations  to  leeward,  varying  in  distance  from 
15J^  to  243^  miles,  nothing  was  heard,  while  at  eleven  sta- 
tions to  windward,  varying  from  8  to  26  miles,  the  sounds 


RECENT  EXPERIMENTS    ON  FOG-SIGNALS  28T 

were  also  inaudible.  It  was  found,  indeed,  that  the  sounds 
proceeding  directly  against  the  wind  did  not  penetrate 
much  beyond  3  miles. 

On  the  following  day,  viz.,  the  18th  October,  we  pro- 
ceeded to  Dungeness  with  the  view  of  making  a  series  of 
strict  comparative  experiments  with  gun-cotton  and  cotton- 
powder.  Eockets  containing  8  oz.,  4  oz.,  and  2  oz.  of 
gun-cotton  had  been  prepared  at  the  Eoyal  Arsenal;  while 
others,  containing  similar  quantities  of  cotton-powder,  had 
been  supplied  by  the  Cotton-powder  Company  at  Faver- 
sham.  With  these  were  compared  the  ordinary  18-pounder 
gun,  which  happened  to  be  mounted  at  Dungeness,  firing 
the  usual  charge  of  3  lbs.  of  powder,  and  a  syren. 

From  these  experiments  it  appeared  that  the  gun-cotton 
and  cotton-powder  were  practically  equal  as  producers  of 
sound. 

The  effectiveness  of  small  charges  was  illustrated  in  a 
very  striking  manner,  only  a  single  unit  separating  the 
numerical  value  of  the  8-oz.  rocket  from  that  of  the  2-oz. 
rocket.  The  former  was  recorded  as  6*9  and  the  latter  as 
5-9,  the  value  of  the  4-oz.  rocket  being  intermediate  be- 
tween them.  These  results  were  recorded  by  a  number 
of  very  practiced  observers  on  board  the  "Galatea." 
They  were  completely  borne  out  by  the  observations  of 
the  Coastguard,  who  marked  the  value  of  the  8-oz.  rocket 
6-1,  and  that  of  the  2-oz.  rocket  5-2.  The  18-pounder 
gun  fell  far  behind  all  the  rockets,  a  result,  possibly,  to 
be  in  part  ascribed  to  the  imperfection  of  the  powder. 
The  performance  of  the  syren  was,  on  the  whole,  less  sat- 
isfactory than  that  of  the  rocket.  The  instrument  was 
worked,  not  by  steam  of  70  lbs.  pressure,  as  at  the  South 
Foreland,  but  by  compressed  air,   beginning  with  40  lbs. 


288  FRAGMENTS   OF  SCIENCE 

and  ending  with  30  lbs.  pressure.  The  trumpet  was 
pointed  to  windward,  and  in  the  axis  of  the  instrument 
the  sound  was  about  as  effective  as  that  of  the  8-oz. 
rocket.  But  in  a  direction  at  right  angles  to  the  axis, 
and  still  more  in  the  rear  of  this  direction,  the  syren  fell 
very  sensibly  behind  even  the  2-oz.  rocket. 

These  are  the  principal  comparative  trials  made  between 
the  gun-cotton  rocket  and  other  fog-signals;  but  they  are 
not  the  only  ones.  On  the  2d  of  August,  1877,  for  ex- 
ample, experiments  were  made  at  Lundy  Island  with  the 
following  results.  At  2  miles  distant  from  the  firing- 
point,  with  land  intervening,  the  18-pounder,  firing  a  3-lb. 
charge,  was  quite  unheard.  Both  the  4-oz.  rocket  and  the 
8-oz.  rocket,  however,  reached  an  elevation  which  com- 
manded the  acoustic  shadow,  and  yielded  loud  reports. 
When  both  were  in  view  the  rockets  were  still  superior 
to  the  gun.  On  the  6th  of  August,  at  St.  Ann's,  the 
4-oz.  and  8-oz.  rockets  proved  superior  to  the  syren.  On 
the  Shambles  Light-vessel,  when  a  pressure  of  13  lbs.  was 
employed  to  sound  the  syren,  the  rockets  proved  greatly 
superior  to  that  instrument.  Proceeding  along  the  sea 
margin  at  Flamboro*  Head,  Mr.  Edwards  states  that  at  a 
distance  of  IJ  mile,  with  the  18-pounder  previously  used 
as  a  fog-signal  hidden  behind  the  cliffs,  its  report  was 
quite  unheard,  while  the  4-oz.  rocket,  rising  to  an  eleva- 
tion which  brought  it  clearly  into  view,  yielded  a  powerful 
sound  in  the  face  of  an  opposing  wind. 

On  the  evening  of  February  9,  1877,  a  remarkable  series 
of  experiments  were  made  by  Mr.  Prentice  at  Stowmarket 
with  the  gun-cotton  rocket.  From  the  report  with  which 
he  has  kindly  furnished  me  I  extract  the  following  partic- 
ulars.     The   first  column  in   the  annexed   statement  con- 


RECENT  EXPERIMENTS   ON  FOO-SIGNALS 


289 


tains  tlie  name  of  the  place  of  observation,  the  second  its 
distance  from  the  firing-point,  and  the  third  the  result 
observed : 


Stoke  Hill,  Ipswich     .     10  miles 
Melton         .         .         .     15     " 

Pramlingliam       .        .     18     *' 

Stratford.     St.  Andrews  19     " 


Tuddenham.    St. 

Martin 

10 

(< 

Christ  Church  Park     . 

11 

(« 

Nettlestead  Hall 

6 

(( 

Bildestone  . 

6 

». 

Nacton 

14 

it 

Aldboro'      . 

25 

it 

Capel  Mills . 

11 

tt 

Lawford 

15i 

it 

Rockets  clearly  seen  and  sounds  distinctly 
heard  53  seconds  after  the  flash. 

Signals  distinctly  heard.  Thought  at  first 
that  sounds  were  reverberated  from  the 
sea. 

Signals  very  distinctly  heard,  both  in  the 
open  air  and  in  a  closed  room.  "Wind  in 
favor  of  sound. 

Reports  loud ;  startled  pheasants  in  a  cover 
close  by. 

Reports  very  loud ;  rolled  away  like  thunder. 

Report  arrived  a  little  more  than  a  minute 
after  flash. 

Distinct  in  every  part  of  observer's  house. 
Very  loud  in  the  open  air. 

Explosion  very  loud,  wind  against  sound. 

Reports  quite  distinct — mistaken  by  inhabi- 
tants for  claps  of  thunder. 

Rockets  seen  through  a  very  hazy  atmos- 
phere ;   a  rumbling  detonation  heard. 

Reports  heard  within  and  without  the  ob- 
server's house.     "Wind  opposed  to  sound. 

Reports  distinct :  attributed  to  distant  thun- 
der. 


In  the  great  majority  of  these  cases,  the  direction  of 
the  sound  enclosed  a  large  angle  with  the  direction  of  the 
wind.  In  some  cases,  indeed,  the  two  directions  were  at 
right  angles  to  each  other.  It  is  needless  to  dwell  for  a 
moment  on  the  advantage  of  possessing  a  signal  command- 
ing ranges  such  as  these. 

The  explosion  of  substances  in  the  air,  after  having 
been  carried  to  a  considerable  elevation  by  rockets,  is  a 
familiar  performance.  In  1873,  moreover,  the  Board  of 
Trade  proposed  a  light- and- sound  rocket  as  a  signal  of  dis- 

SCIENCE — v — 13 


290  FRAGMENTS   OF  SCIENCE 

tress,  which  proposal  was  subsequently  realized,  but  in  a 
form  too  elaborate  and  expensive  for  practical  use.  The 
idea  of  a  gun-cotton  rocket  fit  for  signalling  in  fogs  is,  I 
believe,  wholly  due  to  Sir  Eichard  Collinson,  the  Deputy 
Master  of  the  Trinity  House.  Thanks  to  the  skilful  aid 
given  by  the  authorities  of  Woolwich,  by  Mr.  Prentice  and 
Mr.  Brock,  that  idea  is  now  an  accomplished  fact ;  a  sig- 
nal of  great  power,  handiness  and  economy  being  thus 
placed  at  the  service  of  our  mariners.  Not  only  may  the 
rocket  be  applied  in  association  with  lighthouses  and  light- 
ships, but  in  the  Navy  also  it  may  be  turned  to  important 
account.  Soon  after  the  loss  of  the  *' Vanguard"  I  vent- 
ured to  urge  upon  an  eminent  naval  ofiicer  the  desirabil- 
ity of  having  an  organized  code  of  fog-signals  for  the 
fleet.  He  shook  his  head  doubtingly,  and  referred  to  the 
difficulty  of  finding  room  for  signal  guns.  The  gun-cotton 
rocket  completely  surmounts  this  difficulty.  It  is  manip- 
ulated with  ease  and  rapidity,  while  its  discharges  may  be 
so  grouped  and  combined  as  to  give  a  most  important 
extension  to  the  voice  of  the  admiral  in  command.  It  is 
needless  to  add  that  at  any  point  upon  our  coasts,  or  upon 
any  other  coast,  where  its  establishment  might  be  desir- 
able, a  fog-signal  station  might  be  extemporized  without 
difficulty. 

I  have  referred  more  than  once  to  the  train  of  echoes 
which  accompanied  the  explosion  of  gun-cotton  in  free  air, 
speaking  of  them  as  similar  in  all  respects  to  those  which 
were  described  for  the  first  time  in  my  Report  on  Fog- 
signals,  addressed  to  the  Corporation  of  Trinity  House  in 
1874.'      To  these  echoes  I  attached  a  fundamental  signifi- 

»  See  also  "Philosophical  Transactions"  for  1874,  p.  183. 


RECENT  EXPERIMENTS   ON   FOG-SIGNALS  291 

cance.  There  was  no  visible  reflecting  surface  from  whidi 
they  could  come.  On  some  days,  with  hardly  a  cloud  in 
the  air  and  hardly  a  ripple  on  the  sea,  they  reached  a 
magical  intensity.  As  far  as  the  sense  of  hearing  could 
judge,  they  came  from  the  body  of  the  air  in  front  of  the 
great  trumpet  which  produced  them.  The  trumpet  blasts 
were  five  seconds  in  duration,  but  long  before  the  blast  had 
ceased  the  echoes  struck  in,  adding  their  strength  to  the 
primitive  note  of  the  trumpet.  After  the  blast  had  ended 
the  echoes  continued,  retreating  further  and  further  from 
the  point  of  observation,  and  finally  dying  away  at  great 
distances.  The  echoes  were  perfectly  continuous  as  long 
as  the  sea  was  clear  of  ships,  "tapering"  by  imperceptible 
gradations  into  absolute  silence.  But  when  a  ship  hap- 
pened to  throw  itself  athwart  the  course  of  the  sound,  the 
echo  from  the  broad  side  of  the  vessel  was  returned  as 
a  shock  which  rudely  interrupted  the  continuity  of  the 
dying  atmospheric  music. 

These  echoes  have  been  ascribed  to  reflection  from  the 
crests  of  the  sea-waves.  But  this  hypothesis  is  negatived 
by  the  fact  that  the  echoes  were  produced  in  great  inten- 
sity and  duration  when  no  waves  existed — when  the  sea, 
in  fact,  was  of  glassy  smoothness.  It  has  been  also  shown 
that  the  direction  of  the  echoes  depended  not  on  that  of 
waves,  real  or  assumed,  but  on  the  direction  of  the  axis 
of  the  trumpet.  Causing  that  axis  to  traverse  an  arc  of 
210°,  and  the  trumpet  to  sound  at  various  points  of  the 
arc,  the  echoes  were  always,  at  all  events  in  calm  weather, 
returned  from  that  portion  of  the  atmosphere  toward  which 
the  trumpet  was  directed.  They  could  not,  under  the  cir- 
cumstances, come  from  the  glassy  sea;  while  both  their 
variation  of  direction   and  their  perfectly  continuous  fall 


iJ92  FRAGMENTS   OF  SCIENCE 

into  silence,  are  irreconcilable  with  the  notion  that  they 
came  from  fixed  objects  on  the  land.  They  came  from 
that  portion  of  the  atmosphere  into  which  the  trumpet 
poured  its  maximum  sound,  and  fell  in  intensity  as  the 
direct  sound  penetrated  to  greater  atmospheric  distances. 

The  day  on  which  our  latest  observations  were  made 
was  particularly  fine.  Before  reaching  Dungeness,  the 
smoothness  of  the  sea  and  the  serenity  of  the  air  caused 
me  to  test  the  echoing  power  of  the  atmosphere.  A  single 
ship  lay  about  half  a  mile  distant  between  us  and  the 
land.  The  result  of  the  proposed  experiment  was  clearly 
foreseen.  It  was  this.  The  rocket  being  sent  up,  it  ex- 
ploded at  a  great  height;  the  echoes  retreated  in  their 
usual  fashion,  becoming  less  and  less  intense  as  the  dis- 
tances of  the  invisible  surfaces  of  reflection  from  the  ob- 
servers increased.  About  five  seconds  after  the  explo- 
sion, a  single  loud  shock  was  sent  back  to  us  from  the 
side  of  the  vessel  lying  between  us  and  the  land.  Obliter- 
ated for  a  moment  by  this  more  intense  echo,  the  aerial 
reverberation  continued  its  retreat,  dying  away  into  silence 
in  two  or  three  seconds  afterward.* 

I  have  referred  to  the  firing  of  an  8-oz.  rocket  from  the 
deck  of  the  '* Galatea*'  on  March  8,  1877,  stating  the  dura- 
tion of  its  echoes  to  be  seven  seconds.  Mr.  Prentice,  who 
was  present  at  the  time,  assured  me  that  in  his  experi- 
ments similar  echoes  had  been  frequently  heard  of  more 
than  twice  this  duration.  The  ranges  of  his  sounds  alone 
would  render  this  result  in  the  highest  degree  probable. 


*  The  echoes  of  the  gun  fired  on  shore  this  day  were  very  brief;  those  of 
the  12- oz.  gun-cotton  rocket  were  12"  and  those  of  the  8-oz.  cotton-powder 
rocket  11"  in  duration. 


RECENT  EXPERIMENTS   ON  FOG-SIGNALS  293 

To  attempt  to  interpret  an  experiment  which  I  have 
not  had  an  opportunity  of  repeating,  is  an  operation  of 
some  risk;  and  it  is  not  without  a  consciousness  of  this 
that  I  refer  here  to  a  result  announced  by  Professor  Jo- 
seph Henry,  which  he  considers  adverse  to  the  notion  of 
aerial  echoes.  He  took  the  trouble  to  point  the  trumpet 
of  a  syren  toward  the  zenith,  and  found  that  when  the 
eyren  was  sounded  no  echo  was  returned.  Now  the  re- 
flecting surfaces  which  give  rise  to  these  echoes  are  for 
the  most  part  due  to  differences  of  temperature  between 
sea  and  air.  If,  through  any  cause,  the  air  above  be 
chilled,  we  have  descending  streams — if  the  air  below 
be  warmed,  we  have  ascending  streams  as  the  initial  cause 
of  atmospheric  flocculence.  A  sound  proceeding  vertically 
does  not  cross  the  streams,  nor  impinge  upon  the  reflect- 
ing surfaces,  as  does  a  sound  proceeding  horizontally 
across  them.  Aerial  echoes,  therefore,  will  not  accompany 
the  vertical  sound  as  they  accompany  the  horizontal  one. 
The  experiment,  as  I  interpret  it,  is  not  opposed  to  the 
theory  of  these  echoes  which  I  have  ventured  to  enun- 
ciate. But,  as  I  have  indicated,  not  only  to  see  but  to 
vary  such  an  experiment  is  a  necessary  prelude  to  grasp- 
ing its  full  significance. 

In  a  paper  published  in  the  **  Philosophical  Transac- 
tions" for  1876,  Professor  Osborne  Reynolds  refers  to 
these  echoes  in  the  following  terms:  "Without  attempt- 
ing to  explain  the  reverberations  and  echoes  which  have 
*been  observed,  I  will  merely  call  attention  to  the  fact  that 
in  no  case  have  I  heard  any  attending  the  reports  of  the 
rockets,*  although  they  seem  to  have  been  invariable  with 

*  These  carried  12  oz.  of  gunpowder,  which  has  been  found  by  Colonel  Fraser 
to  require  an  iron  case  to  produce  an  effective  explosion. 


294  FRAGMENTS   OF  SCIENCE 

the  guns  and  pistols.  These  facts  suggest  that  the  echoes 
are  in  some  way  connected  with  the  direction  given  to  the 
sound.  They  are  caused  by  the  voice,  trumpets,  and  the 
syren,  all  of  which  give  direction  to  the  sound;  but  I  am 
not  aware  that  they  have  ever  been  observed  in  the  case 
of  a  sound  which  has  no  direction  of  greatest  intensity.'* 
The  reference  to  the  voice,  and  other  references  in  his 
paper,  cause  me  to  think  that,  in  speaking  of  echoes, 
Professor  Osborne  Reynolds  and  myself  are  dealing  with 
different  phenomena.  Be  that  as  it  may,  the  foregoing 
observations  render  it  perfectly  certain  that  the  condition 
as  to  direction  here  laid  down  is  not  necessary  to  the 
production  of  the  echoes. 

There  is  not  a  feature  connected  with  the  aerial  echoes 
which  cannot  be  brought  out  by  experiments  in  the  air 
of  the  laboratory.  I  have  recently  made  the  following 
experiment:  A  rectangle,  x  Y  (p.  295),  22  inches  by  12, 
was  crossed  by  twenty-three  brass  tubes  (half  the  number 
would  suffice,  and  only  eleven  are  shown  in  the  figure), 
each  having  a  slit  along  it  from  which  gas  can  issue.  In 
this  way  twenty-three  low  flat  flames  were  obtained.  A 
sounding  reed,  a,  fixed  in  a  short  tube  was  placed  at  one 
end  of  the  rectangle,  and  a  "sensitive  flame,'*  *  /,  at  some 
distance  beyond  the  other  end.  When  the  reed  sounded, 
the  flame  in  front  of  it  was  violently  agitated,  and  roared 
boisterously.  Turning  on  the  gas,  and  lighting  it  as  it 
issued  from  the  slits,  the  air  above  the  flames  became 
BO  heterogeneous  that  the  sensitive  flame  was  instantly 
stilled,  rising  from  a  height  of  6  inches  to  a  height  of  18 
inches.    Here  we  had  the  acoustic  opacity  of  the  air  ia 

*  Fully  described  ia  1117  '^Lectures  on  Sound,"  3d  editioa,  p.  227. 


RECEN7    EXPERIMENTS   ON  FOG-SIGNALS 


295 


front  of  the  South  Foreland  strikingly  imitated.*  Turning 
off  the  gas,  and  removing  the  sensitive  flame  to  fy  some 
distance  behind  the  reed,  it  burned  there  tranquilly, 
though  the  reed  was  sounding.  Again  lighting  the  gas 
as  it  issued  from  the  brass  tubes,  the  sound  reflected  from 


Fig.  9. 

the  heterogeneous  air  threw  the  sensitive  flame  into  vio- 
lent agitation.  Here  we  had  imitated  the  aerial  echoes 
heard  when  standing  behind  the  syren-trumpet  at  the 
South  Foreland.  The  experiment  is  extremely  simple,  and 
m  the  highest  degree  impressive.    "" 


The  explosive  rapidity  of  dynamite  marks  it  as  a  sub- 
stance specially  suitable  for  the  production  of  sound.     Ai 


*  "Lectures  on  Sound."  3d  edition,  p.  268. 


296  FRAGMENTS   OF  SCIENCE 

the  suggestion  of  Professor  Dewar,  Mr.  McKoberts  has 
carried  out  a  series  of  experiments  on  dynamite,  with  ex- 
tremely promising  results.  Immediately  after  the  delivery 
of  the  foregoing  lecture  I  was  informed  that  Mr.  Brock 
proposed  the  employment  of  dynamite  in  the  Collinson 
rocket. 


XI 

ON  THE  STUDY   OF   PHYSICS* 

I  HOLD  in  my  hand  an  uncorrected  proof  of  the  sylla- 
bus of  this  course  of  lectures,  and  the  title  of  the 
present  lecture  is  there  stated  to  be  "On  the  Impor- 
tance of  the  Study  of  Physics  as  a  Means  of  Education." 
The  corrected  proof,  however,  contains  the  title:  "On 
the  Importance  of  the  Study  of  Physics  as  a  Branch  of 
Education."  Small  as  this  editorial  alteration  may  seem, 
the  two  words  suggest  two  radically  distinct  modes  of 
viewing  the  subject  before  us.  The  term  Education  is 
sometimes  applied  to  a  single  faculty  or  organ,  and,  if 
we  know  wherein  the  education  of  a  single  faculty  con- 
sists, this  will  help  us  to  clearer  notions  regarding  the 
education  of  the  sum  of  all  the  faculties  or  of  the  mind. 
When,  for  example,  we  speak  of  the  education  of  the 
voice,  what  do  we  mean?  There  are  certain  membranes 
at  the  top  of  the  windpipe  which  throw  into  vibration 
the  air  forced  between  them  from  the  lungs,  thus  pro- 
ducing musical  sounds.  These  membranes  are,  to  some 
extent,  under  the  control  of  the  will,  and  it  is  found  that 
they  can  be  so  modified  by  exercise  as  to  produce  notes 
of  a  clearer  and  more  melodious  character.     This  exercise 

*  From  a  Lecture  delivered  in  the  Royal  Institution  of  Great  Britain  in  the 
spring  of  1854. 

(297) 


298  FRAGMENTS   OF  SCIENCE 

we  call  the  education  of  the  voice.  We  may  choose  for 
our  exercise  songs  new  or  old,  festive  or  solemn;  the  edu- 
cation of  the  voice  being  the  object  aimed  at,  the  songs 
may  be  regarded  as  the  means  by  which  this  education  is 
accomplished.  I  think  this  expresses  the  state  of  the  case 
more  clearly  than  if  we  were  to  call  the  songs  a  branch 
of  education.  Regarding  also  the  education  of  the  human 
mind  as  the  improvement  and  development  of  the  mental 
faculties,  I  shall  consider  the  study  of  Physics  as  a  means 
toward  the  attainment  of  this  end.  From  this  point  of 
view,  I  degrade  Physics  into  an  implement  of  culture,  and 
this  is  my  deliberate  design. 

The  term  Physics,  as  made  use  of  in  the  present  Lect- 
ure, refers  to  that  portion  of  natural  science  which  lies 
midway  between  astronomy  and  chemistry.  The  former, 
indeed,  is  Physics  applied  to  "masses  of  enormous 
weight,*'  while  the  latter  is  Physics  applied  to  atoms 
and  molecules.  The  subjects  of  Physics  proper  are  there- 
fore those  which  lie  nearest  to  human  perception:  light 
and  heat,  color,  sound,  motion,  the  loadstone,  electrical 
attractions  and  repulsions,  thunder  and  lightning,  rain, 
snow,  dew,  and  so  forth.  Oar  senses  stand  between  these 
phenomena  and  the  reasoning  mind.  "We  observe  the  fact, 
but  are  not  satisfied  with  the  mere  act  of  observation :  the 
fact  must  be  accounted  for — fitted  into  its  position  in  the 
line  of  cause  and  effect.  Taking  our  facts  from  Nature 
we  transfer  them  to  the  domain  of  thought:  look  at  them, 
compare  them,  observe  their  mutual  relations  and  connec- 
tions, and  bringing  them  ever  clearer  before  the  mental 
eye,  finally  alight  upon  the  cause  which  unites  them. 
This  is  the  last  act  of  the  mind,  in  this  centripetal  direc- 
tion— in  its  progress  from  the  multiplicity  of  facts  to  the 


ON   THE   STUDY   OF  PHYSICS  299 

central  cause  on  which  they  depend.  But,  having  guessed 
the  cause,  we  are  not  yet  contented.  We  set  out  from  the 
centre  and  travel  in  the  other  direction.  If  the  guess  be 
true,  certain  consequences  must  follow  from  it,  and  we 
appeal  to  the  law  and  testimony  of  experiment  whether 
the  thing  is  so.  Thus  is  the  circuit  of  thought  completed 
— from  without  inward,  from  multiplicity  to  unity,  and 
from  within  outward,  from  unity  to  multiplicity.  In  thus 
traversing  both  ways  the  line  between  cause  and  effect, 
all  our  reasoning  powers  are  called  into  play.  The  mental 
effort  involved  in  these  processes  may  be  compared  to 
those  exercises  of  the  body  which  invoke  the  co-opera- 
tion of  every  muscle,  and  thus  confer  upon  the  whole 
frame  the  benefits  of  healthy  action. 

The  first  experiment  a  child  makes  is  a  physical  exper- 
iment: the  suction-pump  is  but  an  imitation  of  the  first 
act  of  every  new-born  infant.  Nor  do  I  think  it  calcu- 
lated to  lessen  that  infant's  reverence,  or  to  make  him  a 
worse  citizen,  when  his  riper  experience  shows  him  that 
the  atmosphere  was  his  helper  in  extracting  the  first 
draught  from  his  mother's  breast.  The  child  grows,  but 
is  still  an  experimenter:  he  grasps  at  the  moon,  and  his 
failure  teaches  him  to  respect  distance.  At  length  his  lit- 
tle fingers  acquire  sufficient  mechanical  tact  to  lay  hold  of 
a  spoon.  He  thrusts  the  instrument  into  his  mouth,  hurts 
his  gums,  and  thus  learns  the  impenetrability  of  matter. 
He  lets  the  spoon  fall,  and  jumps  with  delight  to  hear  it 
rattle  against  the  table.  The  experiment  made  by  accident 
is  repeated  with  intention,  and  thus  the  young  student  re- 
ceives his  first  lessons  upon  sound  and  gravitation.  There 
are  pains  and  penalties,  however,  in  the  path  of  the  in- 
quirer: he  is  sure  to  go  wrong,  and  Nature  is  just  as  sure 


800  FRAGMENTS   OF  SCIENCE 

to  inform  him  of  the  fact.  He  falls  downstairs,  burns  his 
fingers,  cuts  his  hand,  scalds  his  tongue,  and  in  this  way 
learns  the  conditions  of  his  physical  well  being.  This  is 
Nature's  way  of  proceeding,  and  it  is  wonderful  what 
progress  her  pupil  makes.  His  enjoyments  for  a  time  are 
physical,  and  the  confectioner's  shop  occupies  the  fore- 
ground of  human  happiness;  but  the  blossoms  of  a  finer 
life  are  already  beginning  to  unfold  themselves,  and  the 
relation  of  cause  and  effect  dawns  upon  the  boy.  He 
begins  to  see  that  the  present  condition  of  things  is  not 
final,  but  depends  upon  one  that  has  gone  before,  and  will 
be  succeeded  by  another.  He  becomes  a  puzzle  to  him- 
self; and,  to  satisfy  his  newly- awakened  curiosity,  asks  all 
manner  of  inconvenient  questions.  The  needs  and  ten- 
dencies of  human  nature  express  themselves  through  these 
early  yearnings  of  the  child.  As  thought  ripens,  he  de- 
sires to  know  the  character  and  causes  of  the  phenomena 
presented  to  his  observation;  and  unless  this  desire  has 
been  granted  for  the  express  purpose  of  having  it  re- 
pressed, unless  the  attractions  of  natural  phenomena  be 
like  the  blush  of  the  forbidden  fruit,  conferred  merely  for 
the  purpose  of  exercising  our  self-denial  in  letting  them 
alone;  we  may  fairly  claim  for  the  study  of  Physics  the 
recognition  that  it  answers  to  an  impulse  implanted  by 
Kature  in  the  constitution  of  man. 

A  few  days  ago,  a  Master  of  Arts,  who  is  still  a 
young  man,  and  therefore  the  recipient  of  a  modern  edu- 
cation, stated  to  me  that  until  he  had  reached  the  age  of 
twenty  years  he  had  never  been  taught  anything  whatever 
regarding  natural  phenomena  or  natural  law.  Twelve 
years  of  his  life  previously  had  been  spent  exclusively 
among  the  ancients.     The  case,  I  regret  to  say,  is  typicaL 


ON   THE  STUDY  OF  PHYSICS  301 

Now,  we  cannot,  without  prejudice  to  humanity,  separate 
the  present  from  the  past.  The  nineteenth  century  strikes 
its  roots  into  the  centuries  gone  by,  and  draws  nutriment 
from  them.  The  world  cannot  afford  to  lose  the  record  of 
any  great  deed  or  utterance;  for  such  are  prolific  through- 
out all  time.  We  cannot  yield  the  companionship  of  our 
loftier  brothers  of  antiquity — of  our  Socrates  and  Cato — 
whose  lives  provoke  us  to  sympathetic  greatness  across  the 
interval  of  two  thousand  years.  As  long  as  the  ancient 
languages  are  the  means  of  access  to  the  ancient  mind, 
they  must  ever  be  of  priceless  value  to  humanity;  but 
surely  these  avenues  might  be  kept  open  without  making 
such  sacrifices  as  that  above  referred  to  universal.  We 
have  conquered  and  possessed  ourselves  of  continents  of 
land,  concerning  which  antiquity  knew  nothing;  and  if 
new  continents  of  thought  reveal  themselves  to  the  ex- 
ploring human  spirit,  shall  we  not  possess  them  also? 
In  these  latter  days,  the  study  of  Physics  has  given  us 
glimpses  of  the  methods  of  Nature  which  were  quite  hid- 
den from  the  ancients,  and  we  should  be  false  to  the 
trust  committed  to  us,  if  we  were  to  sacrifice  the  hopes 
and  aspirations  of  the  Present  out  of  deference  to  the 
Past. 

The  bias  of  my  own  education  probably  manifests  itself 
in  a  desire  I  always  feel  to  seize  upon  every  possible  op- 
portunity of  checking  my  assumptions  and  conclusions  by 
experience.  In  the  present  case,  it  is  true,  your  own  con- 
sciousness might  be  appealed  to  in  proof  of  the  tendency 
of  the  human  mind  to  inquire  into  the  phenomena  pre- 
sented to  it  by  the  senses ;  but  I  trust  you  will  excuse  me 
if,  instead  of  doing  this,  I  take  advantage  of  the  facts 
which  have  fallen  in  my  way  through  life,  referring  to 


802  FRAGMENTS   OF  SCIENCE 

your  judgment  to  decide  whether  such  facts  are  truly 
representative  and  general,  and  not  merely  individual 
and  local. 

At  an  agricultural  college  in  Hampshire,  with  which 
I  was  connected  for  some  time,  and  which  is  now  con- 
verted into  a  school  for  the  general  education  of  youth, 
a  society  was  formed  among  the  boys,  who  met  weekly 
for  the  purpose  of  reading  reports  and  papers  upon  vari- 
ous subjects.  The  society  had  its  president  and  treasurer; 
and  abstracts  of  its  proceedings  were  published  in  a  little 
monthly  periodical  issuing  from  the  school  press.  One  of 
the  most  remarkable  features  of  these  weekly  meetings 
was,  that  after  the  general  business  had  been  concluded 
each  member  enjoyed  the  right  of  asking  questions  on 
any  subject  on  which  he  desired  information.  The  ques- 
tions were  either  written  out  previously  in  a  book,  or,  if 
a  question  happened  to  suggest  itself  during  the  meeting, 
it  was  written  upon  a  slip  of  paper  and  handed  in  to  the 
secretary,  who  afterward  read  all  the  questions  aloud.  A 
number  of  teachers  were  usually  present,  and  they  and 
the  boys  made  a  common  stock  of  their  wisdom  in  fur- 
nishing replies.  As  might  be  expected  from  an  assem- 
blage of  eighty  or  ninety  boys,  varying  from  eighteen  to 
eight  years  old,  many  odd  questions  were  proposed.  To 
the  mind  which  loves  to  detect  in  the  tendencies  of  the 
young  the  instincts  of  humanity  generally,  such  questions 
are  not  without  a  certain  philosophic  interest,  and  I  have 
therefore  thought  it  not  derogatory  to  the  present  course 
of  Lectures  to  copy  a  few  of  them  and  to  introduce  them 
here.     They  run  as  follows: 

What  are  the  duties  of  the  Astronomer  Eoyal? 

What  is  frost? 


ON    THE   STUDY   OF   PHYSICS  803 

Why  are  thunder  and  lightning  more  frequent  in  sum- 
mer than  in  winter  ? 

What  occasions  falling  stars? 

What  is  the  cause  of  the  sensation  called  "pins  and 
needles"  ? 

What  is  the  cause  of  waterspouts  ? 

What  is  the  cause  of  hiccup  ? 

If  a  towel  he  wetted  with  water,  why  does  the  wet  por- 
tion become  darker  than  before  ? 

What  is  meant  hy  Lancashire  witches  ? 

Does  the  dew  rise  or  fall  ? 

What  is  the  principle  of  the  hydraulic  press  ? 

Is  there  more  oxygen  in  the  air  in  summer  than  in 
winter  ? 

What  are  those  rings  which  we  see  round  the  gas  and 
sun? 

What  is  thunder  ? 

How  is  it  that  a  black  hat  can  be  moved  by  forming 
round  it  a  magnetic  circle,  while  a  white  hat  remains  sta- 
tionary ? 

What  is  the  cause  of  perspiration? 

Is  it  true  that  men  were  once  monkeys  ? 

What  is  the  difference  between  the  soul  and  the  mindf 

Is  it  contrary  to  -the  rules  of  Vegetarianism  to  eat 
eggs? 

In  looking  over  these  questions,  which  were  wholly 
unprompted,  and  have  been  copied  almost  at  random  from 
the  book  alluded  to,  we  see  that  many  of  them  are  sug- 
gested directly  by  natural  objects,  and  are  not  such  as 
had  an  interest  conferred  on  them  by  previous  culture. 
Now,  the  fact  is  beyond  the  boy's  control,  and  so  cer- 
tainly is  the  desire  to  know  its  cause.     The  sole  question 


804  FRAGMENTS   OF  SCIENCE 

then  is,  whether  this  desire  is  to  be  gratified  or  not. 
Who  created  the  fact?  Who  implanted  the  desire?  Cer- 
tainly not  man.  Who  then  will  undertake  to  place  him- 
self between  the  desire  and  its  fulfilment,  and  proclaim  a 
divorce  between  them?  Take,  for  example,  the  case  of 
the  wetted  towel,  which  at  first  sight  appears  to  be  one 
of  the  most  unpromising  questions  in  the  list.  Shall  we 
tell  the  proposer  to  repress  his  curiosity,  as  the  subject 
is  improper  for  him  to  know,  and  thus  interpose  our  wis- 
dom to  rescue  the  boy  from  the  consequences  of  a  wish 
which  acts  to  his  prejudice?  Or,  recognizing  the  propri- 
ety of  the  question,  how  shall  we  answer  it?  It  is  im- 
possible to  answer  it  without  reference  to  the  laws  of 
optics — without  making  the  boy  to  some  extent  a  natural 
philosopher.  You  may  say  that  the  effect  is  due  to  the 
reflection  of  light  at  the  common  surface  of  two  media  of 
different  refractive  indices.  But  this  answer  presupposes 
on  the  part  of  the  boy  a  knowledge  of  what  reflection 
and  refraction  are,  or  reduces  you  to  the  necessity  of 
explaining  them. 

On  looking  more  closely  into  the  matter,  we  find  that 
our  wet  towel  belongs  to  a  class  of  phenomena  which 
have  long  excited  the  interest  of  philosophers.  The  towel 
is  white  for  the  same  reason  that  snow  is  white,  that  foam 
is  white,  that  pounded  granite  or  glass  is  white,  and  that 
the  salt  we  use  at  table  is  white.  On  quitting  one  me- 
dium and  entering  another,  a  portion  of  light  is  always 
reflected,  but  on  this  condition — the  media  must  possess 
different  refractive  indices.  Thus,  when  we  immerse  a 
bit  of  glass  in  water,  light  is  reflected  from  the  common 
surface  of  both,  and  it  is  this  light  which  enables  us  to 
see  the  glass.     But  when  a  transparent  solid  is  immersed 


ON   THE  STUDY  OF  PHYSICS  805 

in  a  liquid  of  tlie  same  refractive  index  as  itself,  it  im- 
mediately disappears.  I  remember  once  dropping  the 
eyeball  of  an  ox  into  water;  it  vanished  as  if  by  magic, 
with  the  exception  of  the  crystalline  lens,  and  the  sur- 
prise was  so  great  as  to  cause  a  bystander  to  suppose  that 
the  vitreous  humor  had  been  instantly  dissolved.  This, 
however,  was  not  the  case,  and  a  comparison  of  the  re- 
fractive index  of  the  humcr  with  that  of  water  cleared 
up  the  whole  matter.  The  indices  were  identical,  and 
hence  the  light  pursued  its  way  through  both  as  if  they 
formed  one  continuous  mass. 

In  the  case  of  snow,  powdered  quartz,  or  salt,  we  have 
a  transparent  solid  mixed  with  air.  At  every  transition 
from  solid  to  air,  or  from  air  to  solid,  a  portion  of  light 
is  reflected,  and  this  takes  place  so  often  that  the  light  is 
wholly  intercepted.  Thus  from  the  mixture  of  two  trans- 
parent bodies  we  obtain  an  opaque  one.  Now,  the  case 
of  the  towel  is  precisely  similar.  The  tissue  is  composed 
of  semi-transparent  vegetable  fibres,  with  the  interstices 
between  them  filled  with  air;  repeated  reflection  takes 
place  at  the  limiting  surfaces  of  air  and  fibre,  and  hence 
the  towel  becomes  opaque  like  snow  or  salt.  But  if  we 
fill  the  interstices  with  water,  we  diminish  the  reflection; 
a  portion  of  the  light  is  transmitted,  and  the  darkness  of 
the  towel  is  due  to  its  increased  transparency.  Thus  the 
deportment  of  various  minerals,  such  as  hydrophane  and 
tabasheer,  the  transparency  of  tracing  paper  used  by  engi- 
neers, and  many  other  considerations  of  the  highest  scien- 
tific interest,  are  involved  in  the  simple  inquiry  of  this 
unsuspecting  little  boy. 

Again,  take  the  question  regarding  the  rising  or  falling 
of  the  dew — a  question  long  agitated,  and  finally  set  at 


306  FRAGMENTS   OF  SCIENCE 

rest  by  the  beautiful  researches  of  Wells.  I  do  not  think 
that  any  boy  of  average  intelligence  will  be  satisfied  with 
the  simple  answer  that  the  dew  falls.  He  will  wish  to 
learn  how  you  know  that  it  falls,  and,  if  acquainted  with 
the  notions  of  the  middle  ages,  he  may  refer  to  the  opin- 
ion of  Father  Laurus,  that  a  goose  egg,  filled  in  the  morn- 
ing with  dew  and  exposed  to  the  sun,  will  rise  like  a 
balloon — a  swan's  egg  being  better  for  the  experiment 
than  a  goose  egg.  It  is  impossible  to  give  the  boy  a 
clear  notion  of  the  beautiful  phenomenon  to  which  his 
question  refers  without  first  making  him  acquainted  with 
the  radiation  and  conduction  of  heat.  Take,  for  example, 
a  blade  of  grass,  from  which  one  of  these  orient  pearls  is 
depending.  During  the  day  the  grass,  and  the  earth  be- 
neath it,  possess  a  certain  amount  of  warmth  imparted  by 
the  sun;  during  a  serene  night,  heat  is  radiated  from  the 
surface  of  the  grass  into  space,  and,  to  supply  the  loss, 
there  is  a  flow  of  heat  from  the  earth  to  the  blade.  Thus 
the  blade  loses  heat  by  radiation,  and  gains  heat  by  con- 
duction. Now,  in  the  case  before  us,  the  power  of  radia- 
tion is  great,  whereas  the  power  of  conduction  is  small; 
the  consequence  is  that  the  blade  loses  more  than  it  gains, 
and  hence  becomes  more  and  more  refrigerated.  The  light 
vapor  floating  around  the  surface  so  cooled  is  condensed 
upon  it,  and  there  accumulates  to  form  the  little  pearly 
globe  which  we  call  a  dew-drop. 

Thus  the  boy  finds  the  simple  and  homely  fact  which 
addressed  his  senses  to  be  the  outcome  and  flower  of  the 
deepest  laws.  The  fact  becomes,  in  a  measure,  sanctified 
as  an  object  of  thought,  and  invested  for  him  with  a 
beauty  for  evermore.  He  thus  learns  that  things  which, 
at  first  sight,  seem  to  stand  isolated  and  without  apparent 


ON   THE  STUDY   OF   PHYSICS  307 

brotherhood  in  Nature  are  organically  united,  and  finds 
the  detection  of  such  analogies  a  source  of  perpetual  de- 
light. To  enlist  pleasure  on  the  side  of  intellectual  per- 
formance is  a  point  of  the  utmost  importance;  for  the  ex- 
ercise of  the  mind,  like  that  of  the  body,  depends  for  its 
value  upon  the  spirit  in  which  it  is  accomplished.  Every 
physician  knows  that  something  more  than  mere  mechan- 
ical motion  is  comprehended  under  the  idea  of  healthful 
exercise — that,  indeed,  being  most  healthful  which  makes 
us  forget  all  ulterior  ends  in  the  mere  enjoyment  of  it. 
What,  for  example,  could  be  substituted  for  the  action  of 
the  playground,  where  the  boy  plays  for  the  mere  love 
of  playing,  and  without  reference  to  physiological  laws; 
while  kindly  Nature  accomplishes  her  ends  unconsciously, 
and  makes  his  very  indifference  beneficial  to  him.  You 
may  have  more  systematic  motions,  you  may  devise  means 
for  the  more  perfect  traction  of  each  particular  muscle, 
but  you  cannot  create  the  joy  and  gladness  of  the  game, 
and  where  these  are  absent,  the  charm  and  the  health  of 
the  exercise  are  gone.  The  case  is  similar  with  the  edu- 
cation of  the  mind. 

The  study  of  Physics,  as  already  intimated,  consists  of 
two  processes,   which  are   complementary  to   each  other — 
the  tracing   of   facts  to   their  causes,   and   the   logical  ad 
vance  from  the  cause  to  the  fact.     In  the  former  process 
called   induction,  certain   moral   qualities   come  into  play 
The  first  condition  of  success  is  patient  industry,  an  hon 
est  receptivity,   and  a  willingness  to  abandon  all  precon 
ceived   notions,    however  cherished,  if   they   be   found   to 
contradict    the    truth.      Believe    me,    a    self-renunciation 
which     has    something    lofty    in    it,    and    of     which    the 
world  never  hears,   is   often  enacted    in  the   private   ex- 


808  FRAGMENTS   OF  SCIENCE 

perience  of  the  true  votary  of  science.  And  if  a  man 
be  not  capable  of  this  self-renunciation — this  loyal  sur- 
render of  himself  to  Nature  and  to  fact — he  lacks,  in  my 
opinion,  the  first  mark  of  a  true  philosopher.  Thus  the 
earnest  prosecutor  of  science,  who  does  not  work  with 
the  idea  of  producing  a  sensation  in  the  world,  who  loves 
the  truth  better  than  the  transitory  blaze  of  to-day's  fame, 
who  comes  to  his  task  with  a  single  eye,  finds  in  that 
task  an  indirect  means  of  the  highest  moral  culture.  And 
although  the  virtue  of  the  act  depends  upon  its  privacy, 
this  sacrifice  of  self,  this  upright  determination  to  accept 
the  truth,  no  matter  how  it  may  present  itself — even  at 
the  hands  of  a  scientific  foe,  if  necessary — carries  with  it 
its  own  reward.  When  prejudice  is  put  under  foot  and 
the  stains  of  personal  bias  have  been  washed  away — when 
a  man  consents  to  lay  aside  his  vanity  and  to  become  Nat- 
ure's organ — ^his  elevation  is  the  instant  consequence  of 
his  humility.  I  should  not  wonder  if  my  remarks  pro- 
voked a  smile,  for  they  seem  to  indicate  that  I  regard  the 
man  of  science  as  a  heroic,  if  not  indeed  an  angelic,  char- 
acter; and  cases  may  occur  to  you  which  indicate  the  re- 
verse. You  may  point  to  the  quarrels  of  scientific  men, 
to  their  struggles  for  priority,  to  that  unpleasant  egotism 
which  screams  around  its  little  property  of  discovery  like 
a  scared  plover  about  its  young.  I  will  not  deny  all  this; 
but  let  it  be  set  down  to  its  proper  account,  to  the  weak- 
ness— or,  if  you  will — ^to  the  selfishness  of  Man,  but  not 
to  the  charge  of  Physical  Science. 

The  second  process  in  physical  investigation  is  deduc- 
tion^ or  the  advance  of  the  mind  from  fixed  principles  to 
the  conclusions  which  flow  from  them.  The  rules  of  logic 
are  the  formal  statement  of  this  process,  which,  however, 


ON    THE  STUDY  OF  PHYSICS  809 

was  practiced  by  every  healtliy  mind  before  ever  such 
rules  were  written.  In  the  study  of  Physics,  induction 
and  deduction  are  perpetually  wedded  to  each  other.  The 
man  observes,  strips  facts  of  their  peculiarities  of  form, 
and  tries  to  unite  them  by  their  essences :  having  effected 
this,  he  at  once  deduces,  and  thus  checks  his  induction. 
Here  the  grand  difference  between  the  methods  at  present 
followed,  and  those  of  the  ancients,  becomes  manifest. 
They  were  one-sided  in  these  matters:  they  omitted  the 
process  of  induction,  and  substituted  conjecture  for  ob- 
servation. They  could  never,  therefore,  fulfil  the  mission 
of  Man  to  "replenish  the  earth,  and  subdue  it."  The 
subjugation  of  Nature  is  only  to  be  accomplished  by  the 
penetration  of  her  secrets  and  the  patient  mastery  of  her 
laws.  This  not  only  enables  us  to  protect  ourselves  from 
the  hostile  action  of  natural  forces,  but  makes  them  our 
slaves.  By  the  study  of  Physics  we  have  indeed  opened 
to  us  treasuries  of  power  of  which  antiquity  never 
dreamed.  But  while  we  lord  it  over  Matter,  we  have 
thereby  become  better  acquainted  with  the  laws  of  Mind; 
for  to  the  mental  philosopher  the  study  of  Physics  fur- 
nishes a  screen  against  which  the  human  spirit  projects  its 
own  image,  and  thus  becomes  capable  of  self-inspection. 

Thus,  then,  as  a  means  of  intellectual  culture,  the  study 
of  Physics  exercises  and  sharpens  observation:  it  brings 
the  most  exhaustive  logic  into  play:  it  compares,  ab- 
stracts, and  generalizes,  and  provides  a  mental  scenery 
appropriate  to  these  processes.  The  strictest  precision  of 
thought  is  everywhere  enforced,  and  prudence,  foresight, 
and  sagacity  are  demanded.  By  its  appeals  to  experiment, 
it  continually  checks  itself,  and  thus  walks  on  a  founda- 
tion of  facts.     Hence  the  exercise  it  invokes  does  not  end 


810  FRAGMENTS   OF  SCIENCE 

in  a  mere  game  of  intellectual  gymnastics,  sncli  as  the 
ancients  delighted  in,  but  tends  to  the  mastery  of  Nature. 
This  gradual  conquest  of  the  external  world,  and  the 
consciousness  of  augmented  strength  which  accompanies 
it,  render  the  study  of  Physics  as  delightful  as  it  is 
important. 

With  regard  to  the  effect  on  the  imagination,  certain 
it  is  that  the  cool  results  of  physical  induction  furnish 
conceptions  which  transcend  the  most  daring  flights  of 
that  faculty.  Take  for  example  the  idea  of  an  all-per- 
vading ether  which  transmits  a  tingle,  so  to  speak,  to  the 
finger  ends  of  the  universe  every  time  a  street  lamp  is 
lighted.  The  invisible  billows  of  this  ether  can  be  meas- 
ured with  the  same  ease  and  certainty  as  that  with  which 
an  engineer  measures  a  base  and  two  angles,  and  from 
these  finds  the  distance  across  the  Thames.  Kow,  it  is 
to  be  confessed  that  there  may  be  just  as  little  poetry  in 
the  measurement  of  an  ethereal  undulation  as  in  that  of 
the  river;  for  the  intellect,  during  the  acts  of  measurement 
and  calculation,  destroys  those  notions  of  size  which  ap- 
peal to  the  poetic  sense.  It  is  a  mistake  to  suppose,  with 
Dr.  Young,  that 

An  undevout  astronomer  is  mad; 

there  being  no  necessary  connection  between  a  devout 
state  of  mind  and  the  observations  and  calculations  of 
a  practical  astronomer.  It  is  not  until  the  man  withdraws 
from  his  calculation,  as  a  painter  from  his  work,  and  thus 
realizes  the  great  idea  on  which  he  has  been  engaged,  that 
imagination  and  wonder  are  excited.  There  is,  I  admit, 
a  possible  danger  here.  If  the  arithmetical  processes  of 
science  be  too  exclusively  pursued,  they  may  impair  the 


ON   THE  STUDY   OF  PHYSICS  311 

imagination,  and  thus  the  study  of  Physios  is  open  to 
the  same  objection  as  philological,  theological,  or  political 
studies,  when  carried  to  excess.  But  even  in  this  case, 
the  injury  done  is  to  the  investigator  himself:  it  does  not 
reach  the  mass  of  mankind.  Indeed,  the  conceptions  fur- 
nished by  his  cold  unimaginative  reckonings  may  furnish 
themes  for  the  poet,  and  excite  in  the  highest  degree  that 
sentiment  of  wonder  which,  notwithstanding  all  its  foolish 
vagaries,  table-turning  included,  I,  for  my  part,  should 
be  sorry  to  see  banished  from  the  world. 

I  have  thus  far  dwelt  upon  the  study  of  Physics  as 
an  agent  of  intellectual  culture;  but,  like  other  things 
in  Nature,  this  study  subserves  more  than  a  single  end. 
The  colors  of  the  clouds  delight  the  eye,  and,  no  doubt, 
accomplish  moral  purposes  also,  but  the  self- same  clouds 
hold  within  their  fleeces  the  moisture  by  which  our  fields 
are  rendered  fruitful.  The  sunbeams  excite  our  interest 
and  invite  our  investigation;  but  they  also  extend  their 
beneficent  influences  to  our  fruits  and  corn,  and  thus  ac- 
complish, not  only  intellectual  ends,  but  minister,  at  the 
same  time,  to  our  material  necessities.  And  so  it  is  with 
scientific  research.  While  the  love  of  science  is  a  suffi- 
cient incentive  to  the  pursuit  of  science,  and  the  investi- 
gator, in  the  prosecution  of  his  inquiries,  is  raised  above 
all  material  considerations,  the  results  of  his  labors  may 
exercise  a  potent  influence  upon  the  physical  condition 
of  the  community.  This  is  the  arrangement  of  Nature, 
and  not  that  of  the  scientific  investigator  himself;  for  he 
usually  pursues  his  object  without  regard  to  its  practical 
applications. 

And  let  him  who  is  dazzled  by  such  applications — who 
sees   in   the    steam-engine   and   the   electric  telegraph  the 


812  FRAGMENTS   OF  SCIENCE 

highest  embodiment  of  human  genins  and  the  only  legiti- 
mate object  of  scientific  research — beware  of  prescribing 
conditions  to  the  investigator.  Let  him  beware  of  at- 
tempting to  substitute  for  that  simple  love  with  which 
the  votary  of  science  pursues  his  task  the  calculations  of 
what  he  is  pleased  to  call  utility.  The  professed  utili- 
tarian is  unfortunately,  in  most  cases,  the  very  last  man 
to  see  the  occult  sources  from  which  useful  results  are  de- 
rived. He  admires  the  flower,  but  is  ignorant  of  the  con- 
ditions of  its  growth.  The  scientific  man  must  approach 
Nature  in  his  own  way;  for,  if  you  invade  his  freedom 
by  your  so-called  practical  considerations,  it  may  be  at 
the  expense  of  those  qualities  on  which  his  success  as  a 
discoverer  depends.  Let  the  self-styled  practical  man 
look  to  those  from  the  fecundity  of  whose  thought  he, 
and  thousands  like  him,  have  sprung  into  existence. 
Were  they  inspired  in  their  first  inquiries  by  the  cal- 
culations of  utility?  Not  one  of  them.  They  were  often 
forced  to  live  low  and  lie  hard,  and  to  seek  compensation 
for  their  penury  in  the  delight  which  their  favorite  pur- 
suits afforded  them.  In  the  words  of  one  well  qualified 
to  speak  upon  this  subject,  "I  say  not  merely  look  at  the 
pittance  of  men  like  John  Dalton,  or  the  voluntary  star- 
vation of  the  late  Graff;  but  compare  what  is  considered 
as  competency  or  affluence  by  your  Faradays,  Liebigs,  and 
Herschels,  with  the  expected  results  of  a  life  cf  successful 
commercial  enterprise:  then  compare  the  amount  of  mind 
put  forth,  the  work  done  for  society  in  either  case,  and 
you  will  be  constrained  to  allow  that  the  former  belong 
to  a  class  of  workers  who,  properly  speaking,  are  not 
paid,  and  cannot  be  paid  for  their  work,  as  indeed  it 
is  of  a  sort  to  which  no  payment  could  stimulate." 


ON  THE  STUDY  OF  PHYSICS  813 

But  while  the  scientific  investigator,  standing  upon  the 
frontiers  of  human  knowledge,  and  aiming  at  the  conquest 
of  fresh  soil  from  the  surrounding  region  of  the  unknown, 
makes  the  discovery  of  truth  his  exclusive  object  for  the 
time,  he  cannot  but  feel  the  deepest  interest  in  the  prac- 
tical application  of  the  truth  discovered.  There  is  some- 
thing ennobling  in  the  triumph  of  Mind  over  Matter. 
Apart  even  from  its  uses  to  society,  there  is  something 
elevating  in  the  idea  of  Man  having  tamed  that  wild  force 
which  flashes  through  the  telegraphic  wire,  and  made  it 
the  minister  of  his  will.  Our  attainments  in  these  direc- 
tions appear  to  be  commensurate  with  our  needs.  We 
had  already  subdued  horse  and  mule,  and  obtained  from 
them  all  the  service  which  it  was  in  their  power  to  ren- 
der: we  must  either  stand  still,  or  find  more  potent  agents 
to  execute  our  purposes.  At  this  point  the  steam-engine 
appears.  These  are  still  new  things;  it  is  not  long  since 
we  struck  into  the  scientific  methods  which  have  produced 
these  results.  We  cannot  for  an  instant  regard  them  as 
the  final  achievements  of  Science,  but  rather  as  an  ear- 
nest of  what  she  is  yet  to  do.  They  mark  our  first  great 
advances  upon  the  dominion  of  Nature.  Animal  strength 
fails,  but  here  are  the  forces  which  hold  the  world  to- 
gether, and  the  instincts  and  successes  of  Man  assure  him 
that  these  forces  are  his  when  he  is  wise  enough  to  com- 
mand them. 

As  an  instrument  of  intellectual  culture,  the  study  of 
Physics  is  profitable  to  all:  as  bearing  upon  special  func- 
tions, its  value,  though  not  so  great,  is  still  more  tangible. 
"Why,  for  example,  should  Members  of  Parliament  be 
ignorant  of  the  subjects  concerning  which  they  are  called 
upon  to  legislate?     In  this  land  of  practical  physics,  why 

BCIBNOK 14 


814  FRAGMENTS   OF  SCIENCE 

should  they  be  unable  to  form  an  independent  opinion 
upon  a  physical  question?  Why  should  the  member  of 
a  parliamentary  committee  be  left  at  the  mercy  of  inter- 
ested disputants  when  a  scientific  question  is  discussed, 
until  he  deems  the  nap  a  blessing  which  rescues  him  from 
the  bewilderments  of  the  committee  -  room  ?  The  education 
which  does  not  supply  the  want  here  referred  to,  fails  in 
its  duty  to  England.  With  regard  to  our  working  peo- 
ple, in  the  ordinary  sense  of  the  term  working,  the  study 
of  Physics  would,  I  imagine,  be  profitable,  not  only  as  a 
means  of  intellectual  culture,  but  also  as  a  moral  influence 
to  woo  them  from  pursuits  which  now  degrade  them.  A 
man's  reformation  oftener  depends  upon  the  indirect,  than 
upon  the  direct,  action  of  the  will.  The  will  must  be  ex- 
erted in  the  choice  of  employment  which  shall  break  the 
force  of  temptation  by  erecting  a  barrier  against  it.  The 
drunkard,  for  example,  is  in  a  perilous  condition  if  he 
content  himself  merely  with  saying,  or  swearing,  that  he 
will  avoid  strong  drink.  His  thoughts,  if  not  attracted 
by  another  force,  will  revert  to  the  public-house,  and  to 
rescue  him  permanently  from  this,  you  must  give  him 
an  equivalent. 

By  investing  the  objects  of  hourly  intercourse  with  an 
interest  which  prompts  reflection,  new  enjoyments  would 
be  opened  to  the  working  man,  and  every  one  of  these 
would  be  a  point  of  force  to  protect  him  against  tempta- 
tion. Besides  this,  our  factories  and  our  foundries  pre- 
sent an  extensive  field  of  observation,  and  were  those  who 
work  in  them  rendered  capable,  by  previous  culture,  of 
observing  what  they  see,  the  results  might  be  incalculable. 
Who  can  say  what  intellectual  Samsons  are  at  the  present 
moment  toiling  with  closed  eyes  in  the  mills  and  forges 


ON  THE   STUDY  OF  PHYSICS  815 

of  Manchester  and  Birmingliam  ?  Grant  these  Samsons 
sight,  and  you  multiply  the  chances  of  discovery,  and 
with  them  the  prospects  of  national  advancement.  In  our 
multitudinous  technical  operations  we  are  constantly  play- 
ing with  forces  our  ignorance  of  which  is  often  the  cause 
of  our  destruction.  There  are  agencies  at  work  in  a  loco- 
motive of  which  the  maker  of  it  probably  never  dreamed, 
but  which  nevertheless  may  be  sufficient  to  convert  it  into 
an  engine  of  death.  When  we  reflect  on  the  intellectual 
condition  of  the  people  who  work  in  our  coal  mines,  those 
terrific  explosions  which  occur  from  time  to  time  need  not 
astonish  us.  If  these  men  possessed  sufficient  physical 
knowledge,  from  the  operatives  themselves  would  prob- 
ably emanate  a  system  by  which  these  shocking  accidents 
might  be  avoided.  Possessed  of  the  knowledge,  their  per- 
sonal interests  would  furnish  the  necessary  stimulus  to  its 
practical  application,  and  thus  two  ends  would  be  served 
at  the  same  time — the  elevation  of  the  men  and  the  dimi- 
nution of  the  calamity. 

Before  the  present  Course  of  Lectures  was  publicly 
announced,  I  had  many  misgivings  as  to  the  propriety  of 
my  taking  a  part  in  them,  thinking  that  my  place  might 
be  better  filled  by  an  older  and  more  experienced  man. 
To  my  experience,  however,  such  as  it  was,  I  resolved 
to  adhere,  and  I  have  therefore  described  things  as  they 
revealed  themselves  to  my  own  eyes,  and  have  been  en- 
acted in  my  own  limited  practice.  There  is  one  mind 
common  to  us  all;  and  the  true  expression  of  this  mind, 
even  in  small  particulars,  will  attest  itself  by  the  response 
which  it  calls  forth  in  the  convictions  of  my  hearers.  I 
ask  your  permission  to  proceed  a  little  further  in  this 
fashion,  and  to  refer  to  a  fact  or  two  in  addition  to  those 


816  FRAGMENTS    OF  SCIENCE 

already  cited,  which  presented  themselves  to  my  notice 
during  my  brief  career  as  a  teacher  in  the  college  already 
alluded  to.  The  facts,  though  extremely  humble,  and 
deviating  in  some  slight  degree  from  the  strict  subject 
of  the  present  discourse,  may  yet  serve  to  illustrate  an 
educational  principle. 

One  of  the  duties  which  fell  to  my  share  was  the  in- 
struction of  a  class  in  mathematics,  and  I  usually  found 
that  Euclid  and  the  ancient  geometry  generally,  when 
properly  and  sympathetically  addressed  to  the  understand- 
ing, formed  a  most  attractive  study  for  youth.  But  it 
was  my  habitual  practice  to  withdraw  the  boys  from  the 
routine  of  the  book,  and  to  appeal  to  their  self -power  in 
the  treatment  of  questions  not  comprehended  in  that  rou- 
tine. At  first,  the  change  from  the  beaten  track  usually 
excited  aversion:  the  youth  felt  like  a  child  amid  stran- 
gers; but  in  no  single  instance  did  this  feeling  continue. 
When  utterly  disheartened,  I  have  encouraged  the  boy  by 
the  anecdote  of  Newton,  where  he  attributes  the  differ- 
ence between  him  and  other  men  mainly  to  his  own  pa- 
tience; or  of  Mirabeau,  when  he  ordered  his  servant,  who 
had  stated  something  to  be  impossible,  never  again  to  use 
that  blockhead  of  a  word.  Thus  cheered,  the  boy  has  re- 
turned to  his  task  with  a  smile,  which  perhaps  had  some- 
thing of  doubt  in  it,  but  which,  nevertheless,  evinced  a 
resolution  to  try  again.  I  have  seen  his  eye  brighten, 
and,  at  length,  with  a  pleasure  of  which  the  ecstasy  of 
Archimedes  was  but  a  simple  expansion,  heard  him  ex- 
claim, *'I  have  it,  sir."  The  consciousness  of  self -power, 
thus  awakened,  was  of  immense  value;  and,  animated  by 
it,  the  progress  of  the  class  was  astonishing.  It  was  often 
my  custom  to  give  the  boys  the  choice  of  pursuing  their 


ON  THE  STUDY   OF  PHYSIC3  817 

propositions  in  the  book,  or  of  trying  their  strength  at 
others  not  to  be  found  there.  Never  in  a  single  instance 
was  the  book  chosen.  I  was  ever  ready  to  assist  when 
help  was  needful,  but  my  offers  of  assistance  were  habit- 
ually declined.  The  boys  had  tasted  the  sweets  of  intel- 
lectual conquest  and  demanded  victories  of  their  own. 
Their  diagrams  were  scratched  on  the  walls,  cut  into  the 
beams  upon  the  playground,  and  numberless  other  illus- 
trations were  afforded  of  the  living  interest  they  took  in 
the  subject.  For  my  own  part,  as  far  as  experience  in 
teaching  goes,  I  was  a  mere  fledgling — knowing  nothing 
of  the  rules  of  pedagogics,  as  the  Germans  name  it;  but 
adhering  to  the  spirit  indicated  at  the  commencement  of 
this  discourse,  and  endeavoring  to  make  geometry  a  means 
rather  than  a  branch  of  education.  The  experiment  was 
successful,  and  some  of  the  most  delightful  hours  of  my 
existence  have  been  spent  in  marking  the  vigorous  and 
cheerful  expansion  of  mental  power,  when  appealed  to  in 
the  manner  here  described. 

Our  pleasure  was  enhanced  when  we  applied  our  math- 
ematical knowledge  to  the  solution  of  physical  problems. 
Many  objects  of  hourly  contact  had  thus  a  new  interest 
and  significance  imparted  to  them.  The  swing,  the  see- 
saw, the  tension  of  the  giant-stride  ropes,  the  fall  and 
rebound  of  the  football,  the  advantage  of  a  small  boy 
over  a  large  one  when  turning  short,  particularly  in  slip- 
pery weather;  all  became  subjects  of  investigation.  A 
lady  stands  before  a  looking-glass  of  her  own  height;  it 
was  required  to  know  how  much  of  the  glass  was  really 
useful  to  her?  We  learned  with  pleasure  the  economic 
fact  that  she  might  dispense  with  the  lower  half  and  see 
her  whole  figure  notwithstanding.      It  was  also  pleasant 


318  FRAGMENTS   OF  SCIENCE 

to  prove  by  mathematics,  and  verify  by  experiment,  that 
the  angular  velocity  of  a  reflected  beam  is  twice  that  of 
the  mirror  which  reflects  it.  From  the  hum  of  a  bee  we 
were  able  to  determine  the  number  of  times  the  insect 
flaps  its  wings  in  a  second.  Following  up  our  researches 
upon  the  pendulum,  we  learned  how  Colonel  Sabine  had 
made  it  the  means  of  determining  the  figure  of  the  earth; 
and  we  were  also  startled  by  the  inference  which  the 
pendulum  enabled  us  to  draw,  that  if  the  diurnal  velocity 
of  the  earth  were  seventeen  times  its  present  amount,  the 
centrifugal  force  at  the  equator  would  be  precisely  equal 
to  the  force  of  gravitation,  so  that  an  inhabitant  of  those 
regions  would  then  have  the  same  tendency  to  fall  upward 
as  downward.  All  these  things  were  sources  of  wonder 
and  delight  to  us:  and  when  we  remembered  that  we 
were  gifted  with  the  powers  which  had  reached  such 
results,  and  that  the  same  great  field  was  ours  to  work 
in,  our  hopes  arose  that  at  some  future  day  we  might 
possibly  push  the  subject  a  little  further,  and  add  our 
own  victories  to  the  conquests  already  won. 

I  ought  to  apologize  to  you  for  dwelling  so  long  upon 
this  subject;  but  the  days  spent  among  these  young  phi- 
losophers made  a  deep  impression  on  me.  I  learned 
among  them  something  of  myself  and  of  human  nature, 
and  obtained  some  notion  of  a  teacher's  vocation.  If 
there  be  one  profession  in  England  of  paramount  impor- 
tance, I  believe  it  to  be  that  of  the  schoolmaster;  and 
if  there  be  a  position  where  selfishness  and  incompetence 
do  most  serious  mischief,  by  lowering  the  moral  tone  and 
exciting  irreverence  and  cunning  where  reverence  and 
noble  truthfulness  ought  to  be  the  feelings  evoked,  it 
is  that  of   the   principal   of   a   school.      When  a  man   of 


ON   THE   STUDY  OF  PHYSICS  819 

enlarged  heart  and  mind  comes  among  boys — when  he 
allows  his  spirit  to  stream  through  them,  and  observes  the 
operation  of  his  own  character  evidenced  in  the  elevation 
of  theirs — it  would  be  idle  to  talk  of  the  position  of  such 
a  man  being  honorable.  It  is  a  blessed  position.  The 
man  is  a  blessing  to  himself  and  to  all  around  him. 
Such  men,  I  believe,  are  to  be  found  in  England,  and 
it  behooves  those  who  busy  themselves  with  the  mechan- 
ics of  education  at  the  present  day  to  seek  them  out. 
For  no  matter  what  means  of  culture  may  be  chosen, 
whether  physical  or  philological,  success  must  ever  mainly 
depend  upon  the  amount  of  life,  love,  and  earnestness 
which  the  teacher  himself  brings  with  him  to  his  vocation. 
Let  me  again,  and  finally,  remind  you  that  the  claims 
of  that  science  which  finds  in  me  to-day  its  unripened 
advocate  are  those  of  the  logic  of  Nature  upon  the  reason 
of  her  child — ^that  its  disciplines,  as  an  agent  of  culture, 
are  based  upon  the  natural  relations  subsisting  between 
Man  and  the  universe  of  which  he  forms  a  part.  On  the 
one  side,  we  have  the  apparently  lawless  shifting  of  phe- 
nomena; on  the  other  side,  mind,  which  requires  law  for 
its  equilibrium,  and  through  its  own  indestructible  in- 
stincts, as  well  as  through  the  teachings  of  experience, 
knows  that  these  phenomena  are  reducible  to  law.  To 
chasten  this  apparent  chaos  is  a  problem  which  man  has 
set  before  him.  The  world  was  built  in  order:  and  to  us 
are  trusted  the  will  and  power  to  discern  its  harmonies, 
and  to  make  them  the  lessons  of  our  lives.  From  the 
cradle  to  the  grave  we  are  surrounded  with  objects  which 
provoke  inquiry.  Descending  for  a  moment  from  this 
high  plea  to  considerations  which  lie  closer  to  us  as  a 
nation — as  a  land  of  gas  and  furnaces,  of  steam  and  elec- 


320  FRAGMENTS   OF  SCIENCE 

tricity:  as  a  land  which  science,  practically  applied,  has 
made  great  in  peace  and  mighty  in  war — I  ask  you 
whether  this  **land  of  old  and  just  renown'*  has  not  a 
right  to  expect  from  her  institutions  a  culture  more  in 
accordance  with  her  present  needs  than  that  supplied  by 
declension  and  conjugation?  And  if  the  tendency  should 
be  to  lower  the  estimate  of  science,  by  regarding  it  ex- 
clusively as  the  instrument  of  material  prosperity,  let  it 
be  the  high  mission  of  our  universities  to  furnish  the 
proper  counterpoise  by  pointing  out  its  nobler  uses — lift- 
ing the  national  mind  to  the  contemplation  of  it  as  the 
last  development  of  that  "increasing  purpose'*  which  runs 
through  the  ages  and  widens  the  thoughts  of  men. 


XII 

ON    CRYSTALLINE    AND    SLATY    CLEAVAGE* 

WHEN  the  student  of  physical  science  has  to  in- 
vestigate the  character  of  any  natural  force,  his 
first  care  must  be  to  purify  it  from  the  mixture 
of  other  forces,  and  thus  study  its  simple  action.  If,  for 
example,  he  wishes  to  know  how  a  mass  of  liquid  would 
shape  itseK  if  at  liberty  to  follow  the  bent  of  its  own 
molecular  forces,  he  must  see  that  these  forces  have  free 
and  undisturbed  exercise.  We  might  perhaps  refer  him 
to  the  dew-drop  for  a  solution  of  the  question;  but  here 
we  have  to  do,  not  only  with  the  action  of  the  molecules 
of  the  liquid  upon  each  other,  but  also  with  the  action  of 
gravity  upon  the  mass,  which  pulls  the  drop  downward 
and  elongates  it.  If  he  would  examine  the  problem  in 
its  purity,  he  must  do  as  Plateau  has  done — detach  the 
liquid  mass  from  the  action  of  gravity;  he  would  then 
find  the  shape  to  be  a  perfect  sphere.  Natural  processes 
come  to  us  in  a  mixed  manner,  and  to  the  uninstructed 
mind  are  a  mass  of  unintelligible  confusion.  Suppose 
half-a-dozen  of  the  best  musical  performers  to  be  placed 
in  the  same  room,  each  playing  his  own  instrument  to 
perfection,  but  no  two  playing  the  same  tune;  though 
each  individual  instrument  might  be  a  source  of  perfect 

'  From  a  discourse  delivered  in  the  Bojal  Institution  of  Great  Britain, 
June  6,  1866. 

(321) 


822  FRAGMENTS   OF  SCIENCE 

music,  still  the  mixture  of  all  would  produce  mere  noise. 
Thus  it  is  with  the  processes  of  nature,  where  mechanical 
and  molecular  laws  intermingle  and  create  apparent  con- 
fusion. Their  mixture  constitutes  what  may  be  called  the 
noise  of  natural  laws,  and  it  is  the  vocation  of  the  man  of 
science  to  resolve  this  noise  into  its  components,  and  thus 
to  detect  the  underlying  music. 

The  necessity  of  this  detachment  of  one  force  from  all 
other  forces  is  nowhere  more  strikingly  exhibited  than  in 
the  phenomena  of  crystallization.  Here,  for  example,  is  a 
solution  of  common  sulphate  of  soda  or  Glauber  salt. 
Looking  into  it  mentally,  we  see  the  molecules  of  that 
liquid,  like  disciplined  squadrons  under  a  governing  eye, 
arranging  themselves  into  battalions,  gathering  round  dis- 
tinct centres,  and  forming  themselves  into  solid  masses, 
which  after  a  time  assume  the  visible  shape  of  the  crystal 
now  held  in  my  hand.  I  may,  like  an  ignorant  meddler 
wishing  to  hasten  matters,  introduce  confusion  into  this 
order.  This  may  be  done  by  plunging  a  glass  rod  into 
the  vessel;  the  consequent  action  is  not  the  pure  expres- 
sion of  the  crystalline  forces;  the  molecules  rush  together 
with  the  confusion  of  an  unorganized  mob,  and  not  with 
the  steady  accuracy  of  a  disciplined  host.  In  this  mass 
of  bismuth  also  we  have  an  example  of  confused  crystal- 
lization; but  in  the  crucible  behind  me  a  slower  process 
is  going  on:  here  there  is  an  architect  at  work  "who 
makes  no  chips,  no  din,"  and  who  is  now  building  the 
particles  into  crystals,  similar  in  shape  and  structure  to 
those  beautiful  masses  which  we  see  upon  the  table.  By 
permitting  alum  to  crystallize  in  this  slow  way  we  obtain 
these  perfect  octahedrons;  by  allowing  carbonate  of  lime 
to  crystallize,  nature  produces  these  beautiful  rhomboids; 


ON   CRYSTALLINE   AND   SLATY   CLEAVAGE  323 

when  silica  crystallizes,  we  liave  formed  these  hexagonal 
prisms  capped  at  the  ends  by  pyramids;  by  allowing  salt- 
petre to  crystallize  we  have  these  prismatic  masses,  and 
when  carbon  crystallizes  we  have  the  diamond.  If  we 
wish  to  obtain  a  perfect  crystal  we  must  allow  the  molec- 
ular forces  free  play;  if  the  crystallizing  mass  be  per- 
mitted to  rest  upon  a  surface  it  will  be  flattened,  and  to 
prevent  this  a  small  crystal  must  be  so  suspended  as  to 
be  surrounded  on  all  sides  by  the  liquid,  or,  if  it  rest 
upon  the  surface,  it  must  be  turned  daily  so  as  to  present 
all  its  faces  in  succession  to  the  working  builder. 

In  building  up  crystals  these  little  atomic  bricks  often 
arrange  themselves  into  layers  which  are  perfectly  parallel 
to  each  other,  and  which  can  be  separated  by  mechanical 
means;  this  is  called  the  cleavage  of  the  crystal.  The 
crystal  of  sugar  I  hold  in  my  hand  has,  thus  far,  es- 
caped the  solvent  and  abrading  forces  which  sooner  or 
later  determine  the  fate  of  sugar-candy.  I  readily  dis- 
cover that  it  cleaves  with  peculiar  facility  in  one  direc- 
tion. Again  I  lay  my  knife  upon  this  piece  of  rocksalt, 
and  with  a  blow  cleave  it  in  one  direction.  Laying  the 
knife  at  right  angles  to  its  former  position,  the  crystal 
cleaves  again ;  and  finally  placing  the  knife  at  right  angles 
to  the  two  former  positions,  we  find  a  third  cleavage. 
Eocksalt  cleaves  in  three  directions,  and  the  resulting 
solid  is  this  perfect  cube,  which  may  be  broken  up  into 
any  number  of  smaller  cubes.  Iceland  spar  also  cleaves 
in  three  directions,  not  at  right  angles,  but  oblique  to 
each  other,  the  resulting  solid  being  a  rhomboid.  In  each 
of  these  cases  the  mass  cleaves  with  equal  facility  in  all 
three  directions.  For  the  sake  of  completeness  I  may  say 
that  many  crystals  cleave  with  unequal  facility  in  differ 


324  FRAGMENTS   OF  SCIENCE 

ent  directions:  heavy  spar  presents  an  example  of  this 
kind  of  cleavage. 

Turn  we  now  to  the  consideration  of  some  other  phe- 
nomena to  which  the  term  cleavage  may  be  applied. 
Beech,  deal,  and  other  woods  cleave  with  facility  along 
the  fibre,  and  this  cleavage  is  most  perfect  when  the  edge 
of  the  axe  is  laid  across  the  rings  which  mark  the  growth 
of  the  tree.  If  you  look  at  this  bundle  of  hay  severed 
from  a  rick,  you  will  see  a  sort  of  cleavage  in  it  also;  the 
stalks  lie  in  horizontal  planes,  and  only  a  small  force  is 
required  to  separate  them  laterally.  But  we  cannot  re- 
gard the  cleavage  of  the  tree  as  the  same  in  character  as 
that  of  the  hayrick.  In  the  one  case  it  is  the  molecules 
arranging  themselves  according  to  organic  laws  which  pro- 
duce a  cleavable  structure,  in  the  other  case  the  easy 
separation  in  one  direction  is  due  to  the  mechanical 
arrangement  of  the  coarse  sensible  stalks  of  hay. 

This  sandstone  rock  was  once  a  powder  held  in  me- 
chanical suspension  by  water.  The  powder  was  composed 
of  two  distinct  parts — fine  grains  of  sand  and  small  plates 
of  mica.  Imagine  a  wide  strand  covered  by  a  tide,  or  an 
estuary  with  water  which  holds  such  powder  in  suspen- 
sion: how  will  it  sink?  The  rounded  grains  of  sand  will 
reach  the  bottom  first,  because  they  encounter  least  re- 
sistance, the  mica  afterward,  and  when  the  tide  recedes 
we  have  the  little  plates  shining  like  spangles  upon  the 
surface  of  the  sand.  Each  successive  tide  brings  its 
charge  of  mixed  powder,  deposits  its  duplex  layer  day 
after  day,  and  finally  masses  of  immense  thickness  are 
piled  up,  which  by  preserving  the  alternations  of  sand 
and  mica  tell  the  tale  of  their  formation.  Take  the  sand 
and  mica,  mix  them  together  in  water,  and  allow  them  to 


ON  CRYSTALLINE  AND   SLATY  CLEAVAGE  825 

subside;  thej  will  arrange  themselves  in  the  manner  indi- 
cated, and  by  repeating  the  process  jou  can  actually  build 
up  a  mass  which  shall  be  the  exact  counterpart  of  that 
presented  by  Nature.  Now  this  structure  cleaves  with 
readiness  along  the  planes  in  which  the  particles  of  mica 
are  strewn.  Specimens  of  such  a  rock  sent  to  me  from 
Halifax,  and  other  masses  from  the  quarries  of  Over  Dar- 
wen  In  Lancashire,  are  here  before  you.  With  a  hammer 
and  chisel  I  can  cleave  them  into  flags;  indeed,  these 
flags  are  employed  for  roofing  purposes  in  the  districts 
from  which  the  specimens  have  come,  and  receive  the 
name  of  *  *  slatestone. "  But  you  will  discern,  without  a 
word  from  me,  that  this  cleavage  is  not  a  crystalline 
cleavage  any  more  than  that  of  a  hayrick  is.  It  is 
molar,  not  molecular. 

This,  so  far  as  I  am  aware  of,  has  never  been  imag- 
ined, and  it  has  been  agreed  among  geologists  not  to  call 
such  splitting  as  this  cleavage  at  all,  but  to  restrict  the 
term  to  a  phenomenon  of  a  totally  different  character. 

Those  who  have  visited  the  slate  quarries  of  Cumber- 
land and  North  Wales  will  have  witnessed  the  phenome- 
non to  which  I  refer.  We  have  long  drawn  our  supply 
of  roofing-slates  from  such  quarries;  schoolboys  ciphered 
on  these  slates,  they  were  used  for  tombstones  in  church- 
yards, and  for  billiard  tables  in  the  metropolis;  but  not 
until  a  comparatively  late  period  did  men  begin  to  in- 
quire how  their  wonderful  structure  was  produced.  What 
is  the  agency  which  enables  us  to  split  Honister  Crag,  or 
the  cliffs  of  Snowdon,  into  laminae  from  crown  to  base? 
This  question  is  at  the  present  moment  one  of  the  great 
difficulties  of  geologists,  and  occupies  their  attention  per- 
haps  more   than    any  other.      You    may   wonder  at  this. 


826  FRAGMENTS   OF  SCIENCE 

Looking  into  the  quarry  of  Penrhyn,  you  may  be  dis- 
posed to  offer  the  explanation  I  heard  given  two  years 
ago.  *' These  planes  of  cleavage,"  said  a  friend  who  stood 
beside  me  on  the  quarry's  edge,  **are  the  planes  of  strati- 
fication which  have  been  lifted  by  some  convulsion  into 
an  almost  vertical  position."  But  this  was  a  mistake,  and 
indeed  here  lies  the  grand  difficulty  of  the  problem.  The 
planes  of  cleavage  stand  in  most  cases  at  a  high  angle  to 
the  bedding.  Thanks  to  Sir  Koderick  Murchison,  I  am 
able  to  place  the  proof  of  this  before  you.  Here  is  a 
specimen  of  slate  in  which  both  the  planes  of  cleavage 
and  of  bedding  are  distinctly  marked,  one  of  them  mak- 
ing a  large  angle  with  the  other.  This  is  common.  The 
cleavage  of  slates  then  is  not  a  question  of  stratification; 
what  then  is  its  cause? 

In  an  able  and  elaborate  essay  published  in  1835,  Pro- 
fessor Sedgwick  proposed  the  theory  that  cleavage  is  due 
to  the  action  of  crystalline  or  polar  forces  subsequent  to 
the  consolidation  of  the  rock.  "We  may  affirm,"  he  says, 
*'that  no  retreat  of  the  parts,  no  contraction  of  dimensions 
in  passing  to  a  solid  state,  can  explain  such  phenomena. 
They  appear  to  me  only  resolvable  on  the  supposition 
that  crystalline  or  polar  forces  acted  upon  the  whole  mass 
simultaneously  in  one  direction  and  with  adequate  force." 
And  again,  in  another  place:  '* Crystalline  forces  have  re- 
arranged whole  mountain  masses,  producing  a  beautiful 
crystalline  cleavage,  passing  alike  through  all  the  strata. ' '  * 
The  utterance  of  such  a  man  struck  deep,  as  it  ought  to 
do,  into  the  minds  of  geologists,  and  at  the  present  day 
there   are   few   who   do   not   entertain  this   view   either  in 

*  "Transactions  of  the  Greological  Society,'*  ser.  ii.  vol.  iii.  p.  477. 


ON  CRYSTALLINE   AND   SLATY   CLEAVAGE  827 

whole  or  in  part.'  The  boldness  of  the  theory,  indeed, 
has,  in  some  cases,  caused  speculation  to  run  riot,  and  we 
have  books  published  on  the  action  of  polar  forces  and 
geologic  magnetism,  which  rather  astonish  those  who  know 
something  about  the  subject.  According  to  this  theory 
whole  districts  of  North  Wales  and  Cumberland,  moun- 
tains included,  are  neither  more  nor  less  than  the  parts  of 
a  gigantic  crystal.  These  masses  of  slate  were  originally 
fine  mud,  composed  of  the  broken  and  abraded  particles 
of  older  rocks.  They  contain  silica,  alumina,  potash,  soda, 
and  mica  mixed  mechanically  together.  In  the  course  of 
ages  the  mixture  became  consolidated,  and  the  theory  be- 
fore us  assumes  that  a  process  of  crystallization  afterward 
rearranged  the  particles  and  developed  in  it  a  single  plane 
of  cleavage.  Though  a  bold,  and  I  think  inadmissable, 
stretch  of  analogies,  this  hypothesis  has  done  good  ser- 
vice. Right  or  wrong,  a  thoughtfully  uttered  theory  has 
a  dynamic  power  which  operates  against  intellectual  stag- 
nation; and  even  by  provoking  opposition  is  eventually 
of  service  to  the  cause  of  truth.  It  would,  however,  have 
been  remarkable  if,  among  the  ranks  of  geologists  them- 
selves, men  were  not  found  to  seek  an  explanation  of 
slate -cleavage  involving  a  less  hardy  assumption. 

The  first  step  in  an  inquiry  of  this  kind  is  to  seek 


*  In  a  letter  to  Sir  Charles  Lyell,  dated  from  the  Cape  of  Good  Hope,  Febru- 
ary 20,  1836,  Sir  John  Herschel  writes  as  follows:  "If  rocks  have  been  so 
heated  as  to  allow  of  a  commencement  of  crystallization,  that  is  to  say,  if  they 
have  been  heated  to  a  point  at  which  the  particles  can  begin  to  move  among 
themselves,  or  at  least  on  their  own  axes,  some  general  law  must  then  deter- 
mine the  position  in  which  these  particles  will  rest  on  cooling.  Probably  that 
position  will  have  some  relation  to  the  direction  in  which  the  heat  escapes. 
Now,  when  all  or  a  majority  of  particles  of  the  same  nature  have  a  general 
tendency  to  one  position,  that  must  of  course  determine  a  cleavage  plane." 


828  FRAGMENTS    OF  SCIENCE 

facts.  This  has  been  done,  and  the  labors  of  Daniel 
Sharpe  (the  late  President  of  the  Geological  Society,  who, 
to  the  loss  of  science  and  the  sorrow  of  all  who  knew 
him,  has  so  suddenly  been  taken  away  from  ns),  Mr. 
Henry  Clifton  Sorby,  and  others,  have  furnished  us  with 
a  body  of  facts  associated  with  slaty  cleavage,  and  having 
a  most  important  bearing  upon  the  question. 

Fossil  shells  are  found  in  these  slate-rocks.  I  have 
here  several  specimens  of  such  shells  in  the  actual  rock, 
and  occupying  various  positions  in  regard  to  the  cleavage 
planes.  They  are  squeezed,  distorted,  and  crushed;  in  all 
cases  the  distortion  leads  to  the  inference  that  the  rock 
which  contains  these  shells  has  been  subjected  to  enor- 
mous pressure  in  a  direction  at  right  angles  to  the  planes 
of  cleavage.  The  shells  are  all  flattened  and  spread  out 
in  these  planes.  Compare  this  fossil  trilobite  of  normal 
proportions  with  these  others  which  have  suffered  dis- 
tortion. Some  have  lain  across,  some  along,  and  some 
oblique  to  the  cleavage  of  the  slate  in  which  they  are 
found;  but  in  all  cases  the  distortion  is  such  as  required 
for  its  production  a  compressing  force  acting  at  right 
angles  to  the  planes  of  cleavage.  As  the  trilobites  lay 
in  the  mud,  the  jaws  of  a  gigantic  vice  appear  to  have 
closed  upon  them  and  squeezed  them  into  the  shapes 
you    see. 

We  sometimes  find  a  thin  layer  of  coarse  gritty  ma- 
terial, between  two  layers  of  finer  rock,  through  which 
and  across  the  gritty  layer  pass  the  planes  of  lamination. 
The  coarse  layer  is  found  bent  by  the  pressure  into  sinu- 
osities like  a  contorted  ribbon.  Mr.  Sorby  has  described 
a  striking  case  of  this  kind.  This  crumpling  can  be  ex- 
perimentally imitated;    the  amount  of  compression  might, 


ON  CRYSTALLINE   AND   SLATY   CLEAVAGE         329 

moreover,  be  roughly  estimated  by  supposing  the  con- 
torted bed  to  be  stretched  out,  its  length  measured  and 
compared  with  the  shorter  distance  into  which  it  has 
been  squeezed.  We  find  in  this  way  that  the  yielding 
of  the  mass  has  been  considerable. 

Let  me  now  direct  your  attention  to  another  proof  of 
pressure;  you  see  the  varying  colors  which  indicate  the 
bedding  on  this  mass  of  slate.  The  dark  portion  is  gritty, 
being  composed  of  comparatively  coarse  particles,  which, 
owing  to  their  size,  shape  and  gravity,  sink  first  and  con- 
stitute the  bottom  of  each  layer.  Gradually,  from  bottom 
to  top  the  coarseness  diminishes,  and  near  the  upper  sur- 
face we  have  a  layer  of  exceedingly  fine  grain.  It  is  the 
fine  mud  thus  consolidated  from  which  are  derived  the 
German  razor-stones,  so  much  prized  for  the  sharpening 
of  surgical  instruments.  When  a  bed  is  thin,  the  fine- 
grain  slate  is  permitted  to  rest  upon  a  slab  of  the  coarse 
slate  in  contact  with  it;  when  the  fine  bed  is  thick,  it  is 
cut  into  slices  which  are  cemented  to  pieces  of  ordinary 
slate,  and  thus  rendered  stronger.  The  mud  thus  depos- 
ited is,  as  might  be  expected,  often  rolled  up  into  nodular 
masses,  carried  forward,  and  deposited  among  coarser  ma- 
terial by  the  rivers  from  which  the  slate-mud  has  sub- 
sided. Here  are  such  nodules  enclosed  in  sandstone. 
Everybody,  moreover,  who  has  ciphered  upon  a  school- 
slate  must  remember  the  whitish-green  spots  which  some- 
times dotted  the  surface  of  the  slate,  and  over  which  the 
pencil  usually  slid  as  if  the  spots  were  greasy.  Now  these 
spots  are  composed  of  the  finer  mud,  and  they  could  not, 
on  account  of  their  fineness,  hite  the  pencil  like  the  sur- 
rounding gritty  portions  of  the  slate.  Here  is  a  beautiful 
example  of  these  spots:   you  observe  them,  on  the  cleav- 


880  FRAGMENTS   OF  SCIENCE 

age  surface,  in  broad  round  patches.  But  turn  the  slate 
edgewise  and  the  section  of  each  nodule  is  seen  to  be  a 
sharp  oval  with  its  longer  axis  parallel  to  the  cleavage. 
This  instructive  fact  has  been  adduced  by  Mr.  Sorby.  1 
have  made  excursions  to  the  quarries  of  Wales  and  Cum- 
berland, and  to  many  of  the  slate  yards  of  London,  and 
found  the  fact  general.  Thus  we  elevate  a  common  ex- 
perience of  our  boyhood  into  evidence  of  the  highest  sig- 
nificance as  regards  a  most  important  geological  problem. 
From  the  magnetic  deportment  of  these  slates,  I  was  led 
to  infer  that  these  spots  contain  a  less  amount  of  iron  than 
the  surrounding  dark  slate.  An  analysis  was  made  for 
me  by  Mr.  Hambly  in  the  laboratory  of  Dr.  Percy  at  the 
School  of  Mines  with  the  following  result: 

Analysis  op  Slate 
Dark  Slate,  two  analyses 

1.  Percentage  of  iron 5 '86 

2.  '•  ".......         613 

Mean    .         5  -99 

Whitish  Green  Slate 

1.  Percentage  of  iron 3  24 

2.  "  •* 312 

Mean     .         3  18 

According  to  these  analyses  the  quantity  of  iron  in  the 
dark  slate  immediately  adjacent  to  the  greenish  spot  is 
nearly  double  the  quantity  contained  in  the  spot  itself. 
This  is  about  the  proportion  which  the  magnetic  experi- 
ments suggested. 

Let  me  now  remind  you  that  the  facts  brought  before 
you  are  typical — each  is  the  representative  of  a  class.  We 
have   seen   shells   crushed;    the    trilobites    squeezed,    beds 


ON  CRYSTALLINE   AND   SLATY  CLEAVAGE  331 

contorted,  nodules  of  greenish  marl  flattened;  and  all 
these  sources  of  independent  testimony  point  to  one  and 
the  same  conclusion,  namely,  that  slate-rocks  have  been 
subjected  to  enormous  pressure  in  a  direction  at  right 
angles  to  the  planes  of  cleavage. 

In  reference  to  Mr.  Sorby's  contorted  bed,  I  have  said 
that  by  supposing  it  to  be  stretched  out  and  its  length 
measured,  it  would  give  us  an  idea  of  the  amount  of 
yielding  of  the  mass  above  and  below  the  bed.  Such  a 
measurement,  however,  would  not  give  the  exact  amount 
of  yielding.  I  hold  in  my  hand  a  specimen  of  slate  with 
its  bedding  marked  upon  it;  the  lower  portions  of  each 
layer  being  composed  of  a  comparatively  coarse  gritty  ma- 
terial something  like  what  you  may  suppose  the  contorted 
bed  to  be  composed  of.  Now,  in  crossing  these  gritty  por- 
tions, the  cleavage  turns,  as  if  tending  to  cross  the  bed 
ding  at  another  angle.  When  the  pressure  began  to  act, 
the  intermediate  bed,  which  is  not  entirely  unyielding, 
suffered  longitudinal  pressure;  as  it  bent,  the  pressure  be- 
came gradually  more  transverse,  and  the  direction  of  its 
cleavage  is  exactly  such  as  you  would  infer  from  an  ac- 
tion of  this  kind — it  is  neither  quite  across  the  bed,  nor 
yet  in  the  same  direction  as  the  cleavage  of  the  slate 
above  and  below  it,  but  intermediate  between  both.  Sup- 
posing the  cleavage  to  be  at  right  angles  to  the  pressure, 
this  is  the.  direction  which  it  ought  to  take  across  these 
more  unyielding  strata. 

Thus  we  have  established  the  concurrence  of  the  phe- 
nomena of  cleavage  and  pressure — that  they  accompany 
each  other;  but  the  question  still  remains,  Is  the  pressure 
sufficient  to  account  for  the  cleavage?  A  single  geologist, 
as  far  as  I  am  aware,   answers  boldly  in  the   affirmative. 


332  FRAGMENTS   OF  SCIENCE 

This  geologist  is  Sorby,  who  has  attacked  the  question 
in  the  true  spirit  of  a  physical  investigator.  Call  to 
mind  the  cleavage  of  the  flags  of  Halifax  and  Over  Dar- 
wen,  which  is  caused  by  the  interposition  of  layers  of 
mica  between  the  gritty  strata.  Mr.  Sorby  finds  plates 
of  mica  to  be  also  a  constituent  of  slate-rock.  He  asks 
himself,  what  will  be  the  effect  of  pressure  upon  a  mass 
containing  such  plates  confusedly  mixed  up  in  it?  It  will 
be,  he  argues,  and  he  argues  rightly,  to  place  the  plates 
with  their  flat  surfaces  more  or  less  perpendicular  to  the 
direction  in  which  the  pressure  is  exerted.  He  takes 
scales  of  the  oxide  of  iron,  mixes  them  with  a  fine  pow- 
der, and  on  squeezing  the  mass  finds  that  the  tendency 
of  the  scales  is  to  set  themselves  at  right  angles  to  the 
line  of  pressure.  Along  the  planes  of  weakness  produced 
by  the  scales  the  mass  cleaves. 

By  tests  of  a  different  character  from  those  applied  by 
Mr.  Sorby,  it  might  be  shown  how  true  his  conclusion 
is — that  the  effect  of  pressure  on  elongated  particles,  or 
plates,  will  be  such  as  he  describes  it.  But  while  the 
scales  must  be  regarded  as  a  true  cause,  I  should  not 
ascribe  to  them  a  large  share  in  the  production  of  the 
cleavage.  I  believe  that  even  if  the  plates  of  mica  were 
wholly  absent,  the  cleavage  of  slate-rocks  would  be  much 
the  same  as  it  is  at  present. 

Here  is  a  mass  of  pure  white  wax:  it  contains  no  mica 
particles,  no  scales  of  iron,  or  anything  analogous  to  them. 
Here  is  the  self-same  substance  submitted  to  pressure.  I 
would  invite  the  attention  of  the  eminent  geologists  now 
before  me  to  the  structure  of  this  wax.  No  slate  ever 
exhibited  so  clean  a  cleavage ;  it  splits  into  laminae  of  sur- 
passing tenuity,  and  proves  at  a  single  stroke  that  press- 


ON  CRYSTALLINE   AND  SLATY  CLEAVAGE  333 

ure  is  sufficient  to  produce  cleavage,  and  that  this  cleav- 
age is  independent  of  intermixed  plates  or  scales.  I  have 
purposely  mixed  this  wax  with  elongated  particles,  and 
am  unable  to  say  at  the  present  moment  that  the  cleavage 
is  sensibly  affected  by  their  presence — if  anything,  I  should 
say  they  rather  impair  its  fineness  and  clearness  than  pro- 
mote it. 

The  finer  the  slate  is  the  more  perfect  will  be  the  re- 
semblance of  its  cleavage  to  that  of  the  wax.  Compare 
the  surface  of  the  wax  with  the  surface  of  this  slate  from 
Borrodale  in  Cumberland.  You  have  precisely  the  same 
features  in  both:  you  see  flakes  clinging  to  the  surfaces 
of  each,  which  have  been  partially  torn  away  in  cleaving. 
Let  any  close  observer  compare  these  two  effects,  he  will, 
I  am  persuaded,  be  led  to  the  conclusion  that  they  are 
the  product  of  a  common  cause.* 

But  you  will  ask  me  how,  according  to  my  view,  does 
pressure  produce  this  remarkable  result?  This  may  be 
stated  in  a  very  few  words. 

There  is  no  such  thing  in  nature  as  a  body  of  perfectly 
homogeneous  structure.  I  break  this  clay  which  seems  so 
uniform,  and  find  that  the  fracture  presents  to  my  eyes 
innumerable  surfaces  along  which  it  has  given  way,  and 
it  has  yielded  along  those  surfaces  because  in  them  the 
cohesion  of  the  mass  is  less  than  elsewhere.  I  break  this 
marble,  and  even  this  wax,  and  observe  the  same  result; 
look  at  the  mud  at  the  bottom  of  a  dried  pond;  look  at 


'  I  have  usually  softened  the  wax  by  warming  it,  kneaded  it  with  the  fingers, 
and  pressed  it  between  thick  plates  of  glass  previously  wetted.  At  the  ordinary 
summer  temperature  the  pressed  wax  is  soft,  and  tears  rather  than  cleaves;  on 
this  account  I  cool  my  compressed  specimens  in  a  mixture  of  pounded  ice  and 
salt,  and  when  thus  cooled  they  split  cleanly. 


834  FRAGMENTS   OF  SCIENCE 

some  of  the  ungravelled  walks  in  Kensington  Gardens  on 
drying  after  rain — they  are  cracked  and  split,  and,  other 
circumstances  being  equal,  they  crack  and  split  where  the 
cohesion  is  a  minimum.  Take  then  a  mass  of  partially 
consolidated  mud.  Such  a  mass  is  divided  and  subdivided 
by  interior  surfaces  along  which  the  cohesion  is  compara- 
tively small.  Penetrate  the  mass  in  idea,  and  you  will  see 
it  composed  of  numberless  irregular  polyhedra  bounded 
by  surfaces  of  weak  cohesion.  Imagine  such  a  mass  sub- 
jected to  pressure — ^it  yields  and  spreads  out  in  the  direc- 
tion of  least  resistance ;  *  the  little  polyhedra  become  con- 
verted into  laminae,  separated  from  each  other  by  surfaces 
of  weak  cohesion,  and  the  infallible  result  will  be  a  ten- 
dency to  cleave  at  right  angles  to  the  line  of  pressure. 

Further,  a  mass  of  dried  mud  is  full  of  cavities  and 
fissures.  If  you  break  dried  pipe-clay  you  see  them  in 
great  numbers,  and  there  are  multitudes  of  them  so  small 
that  you  cannot  see  them.  A  flattening  of  these  cavities 
must  take  place  in  squeezed  mud,  and  this  must  to  some 
extent  facilitate  the  cleavage  of  the  mass  in  the  direction 
indicated. 

Although  the  time  at  my  disposal  has  not  permitted 
me  duly  to  develop  these  thoughts,  yet  for  the  last  twelve 
months  the  subject  has  presented  itself  to  me  almost  daily 
under  one  aspect  or  another.  I  have  never  eaten  a  bis- 
cuit during   this   period    without   remarking    the   cleavage 

*  It  is  scarcely  necessary  to  say  that  if  the  mass  were  squeezed  equally  in  all 
directions  no  laminated  structure  could  be  produced :  it  must  have  room  to  yield 
in  a  lateral  direction.  Mr.  Warren  De  la  Rue  informs  me  that  he  once  wished 
to  obtain  white-lead  in  a  fine  granular  slate,  and  to  accomplish  this  he  first  com- 
pressed it.  The  mold  was  conical,  and  permitted  the  lead  to  spread  out  a  little 
laterally.  The  lamination  was  as  perfect  as  that  of  slate,  and  it  quite  defeated 
him  in  his  effort  to  obtain  a  granular  powder. 


ON  CRYSTALLINE   AND   SLATY   CLEAVAGE  835 

developed  by  the  rolling-pin.  You  have  only  to  break  a 
biscuit  across,  and  to  look  at  the  fracture,  to  see  the  lam- 
inated structure.  We  have  here  the  means  of  pushing  the 
analogy  further.  I  invite  you  to  compare  the  structure 
of  this  slate,  which  was  subjected  to  a  high  temperature 
during  the  conflagration  of  Mr.  Scott  Russell's  premises, 
with  that  of  a  biscuit.  Air  or  vapor  within  the  slate  has 
caused  it  to  swell,  and  the  mechanical  structure  it  reveals 
is  precisely  that  of  a  biscuit.  During  these  inquiries  I 
have  received  much  instruction  in  the  manufacture  of 
puff-paste.  Here  is  some  such  paste  baked  under  my  own 
superintendence.  The  cleavage  of  our  hills  is  accidental 
cleavage,  but  this  is  cleavage  with  intention.  The  voli- 
tion of  the  pastry-cook  has  entered  into  its  formation.  It 
has  been  his  aim  to  preserve  a  series  of  surfaces  of  struct- 
ural weakness,  along  which  the  dough  divides  into  layers. 
Puff -paste  in  preparation  must  not  be  handled  too  much; 
it  ought,  moreover,  to  be  rolled  on  a  cold  slab,  to  pre- 
vent the  butter  from  melting,  and  diffusing  itself,  thus 
rendering  the  paste  more  homogeneous  and  less  liable  to 
split.  Puff- paste  is,  then,  simply  an  exaggerated  case  of 
slaty  cleavage. 

The  principle  here  enunciated  is  so  simple  as  to  be 
almost  trivial;  nevertheless,  it  embraces  not  only  the  cases 
mentioned,  but,  if  time  permitted,  it  might  be  shown  you 
that  the  principle  has  a  much  wider  range  of  application. 
When  iron  is  taken  from  the  puddling  furnace  it  is  more 
or  less  spongy,  an  aggregate  in  fact  of  small  nodules:  it 
is  at  a  welding  heat,  and  at  this  temperature  is  submitted 
to  the  process  of  rolling.  Bright,  smooth  bars  are  the 
result.  But  notwithstanding  the  high  heat  the  nodules 
do  not  perfectly  blend  together.      The  process  of  rolling 


836  FRAGMENTS   OF  SCIENCE 

draws  them  into  fibres.  Here  is  a  mass  acted  upon  by 
dilute  sulphuric  acid,  which  exhibits  in  a  striking  manner 
this  fibrous  structure.  The  experiment  was  made  by  my 
friend  Dr.  Percy,  without  any  reference  to  the  question 
of  cleavage. 

Break  a  piece  of  ordinary  iron  and  you  have  a  granu- 
lar fracture;  beat  the  iron,  you  elongate  these  granules, 
and  finally  render  the  mass  fibrous.  Here  are  pieces  of 
rails  along  which  the  wheels  of  locomotives  have  slidden; 
the  granules  have  yielded  and  become  plates.  They  ex- 
foliate or  come  off  in  leaves;  all  these  effects  belong,  I 
believe,  to  the  great  class  of  phenomena  of  which  slaty 
cleavage  forms  the  most  prominent  example.* 

We  have  now  reached  the  termination  of  our  task. 
You  have  witnessed  the  phenomena  of  crystallization,  and 
have  had  placed  before  you  the  facts  which  are  found 
associated  with  the  cleavage  of  slate  rocks.  Such  facts, 
as  expressed  by  Helmholtz,  are  so  many  telescopes  to  our 
spiritual  vision,  by  which  we  can  see  backward  through 
the  night  of  antiquity,  and  discern  the  forces  which  have 
been  in  operation  upon  the  earth's  surface 

Ere  the  lion  roared. 
Or  the  eagle  soared. 

From  evidence  of  the  most  independent  and  trust- 
worthy character,  we  come  to  the  conclusion  that  these 
slaty  masses  have  been  subjected  to  enormous  pressure, 
and  by  the  sure  method  of  experiment  we  have  shown — 
and  this   is   the  only  really  new  point  which    has   been 


*  For  some  further  observations  on  this  subject,  by  Mr.  Sorby  and  myself ^ 
"Philosophical  Magazine"  for  August,  1856. 


ON   CRYSTALLINE   AND    SLATY   CLEAVAGE         387 

brought  before  you — bow  tbe  pressure  is  sufficient  to  pro- 
duce the  cleavage.  ICxpanding  our  field  of  view,  we  find 
tbe  self- same  law,  whose  footsteps  we  trace  amid  the  crags 
of  Wales  and  Cumberland,  extending  into  the  domain  of 
the  pastry-cook  and  iron-founder;  nay,  a  wheel  cannot  roll 
over  the  half- dried  mud  of  our  streets  without  revealing 
to  us  more  or  less  of  the  features  of  this  law.  Let  me 
say,  in  conclusion,  that  the  spirit  in  which  this  problem 
has  been  attacked  by  geologists  indicates  the  dawning  of 
a  new  day  for  their  science.  The  great  intellects  who 
have  labored  at  geology,  and  who  have  raised  it  to  its 
present  pitch  of  grandeur,  were  compelled  to  deal  with 
the  subject  in  mass;  they  had  no  time  to  look  after  de- 
tails. But  the  desire  for  more  exact  knowledge  is  increas- 
ing; facts  are  flowing  in  which,  while  they  leave  un- 
touched the  intrinsic  wonders  of  geology,  are  gradually 
supplanting  by  solid  truths  the  uncertain  speculations 
which  beset  the  subject  in  its  infancy.  Geologists  now 
aim  to  imitate,  as  far  as  possible,  the  conditions  of 
nature,  and  to  produce  her  results;  they  are  approaching 
more  and  more  to  the  domain  of  physics,  and  I  trust  the 
day  will  soon  come  when  we  shall  interlace  our  friendly 
arms  across  the  common  boundary  of  our  sciences,  and 
pursue  our  respective  tasks  in  a  spirit  of  mutual  helpful- 
ness, encouragement  and  goodwill. 

[I  would  now  lay  more  stress  on  the  lateral  yielding, 
referred  to  in  the  note  at  the  bottom  of  page  384,  accom- 
panied as  it  is  by  tangential  sliding,  than  I  was  prepared 
to  do  when  this  Lecture  was  given.  This  sliding  is,  I 
think,  the  principal  cause  of  the  planes  of  weakness,  both 
in  pressed  wax  and  slate  rock.     J.  T.,  1871.] 


XIII 

ON    PARAMAGNETIC    AND    DIAMAGNETIC    FORCES* 

THE  notion  of  an  attractive  force,  which  draws  bodies 
toward  the  centre  of  the  earth,  was  entertained  by 
Anaxagoras  and  his  pupils,  bj  Democritus,  Pythag- 
oras, and  Epicurus;  and  the  conjectures  of  these  ancients 
were  renewed  by  Galileo,  Huyghens,  and  others,  who 
stated  that  bodies  attract  each  other  as  a  magnet  attracts 
iron.  Kepler  applied  the  notion  to  bodies  beyond  the 
surface  of  the  earth,  and  affirmed  the  extension  of  this 
force  to  the  most  distant  stars.  Thus  it  would  appear 
that  in  the  attraction  of  iron  by  a  magnet  originated  the 
conception  of  the  force  of  gravitation.  Nevertheless,  if 
we  look  closely  at  the  matter,  it  will  be  seen  that  the 
magnetic  force  possesses  characters  strikingly  distinct  from 
those  of  the  force  which  holds  the  universe  together.  The 
theory  of  gravitation  is,  that  every  particle  of  matter  at- 
tracts every  other  particle;  in  magnetism  also  we  have 
attraction,  but  we  have  always,  at  the  same  time,  repul- 
sion, the  final  effect  being  due  to  the  difference  of  these 
two  forces.  A  body  may  be  intensely  acted  on  by  a 
magnet,  and  still  no  motion  of  translation  will  follow,  if 
the  repulsion  be  equal  to  the  attraction.     Previous  to  mag- 


*  Abstract  of  a  discourse  delivered  in  the  Royal  Institution  of  Great  Britain, 
February  1,  1856. 
(338) 


PARAMAGNETIC    AND    DIAMAONETJC   FORCES       339 

netization,  a  clipping  needle,  when  its  centre  of  gravity  is 
supported,  stands  accurate Ij  level;  but,  after  magnetiza- 
tion, one  end  of  it,  in  our  latitude,  is  pulled  toward  the 
north  pole  of  the  earth.  The  needle,  however,  being  sus- 
pended from  the  arm  of  a  fine  balance,  its  weight  is  found 
unaltered  by  its  magnetization.  In  like  manner,  when  the 
needle  is  permitted  to  float  upon  a  liquid,  and  thus  to 
follow  the  attraction  of  the  north  magnetic  pole  of  the 
earth,  there  is  no  motion  of  the  mass  toward  that  pole. 
The  reason  is  known  ^  to  be,  that  although  the  marked 
end  of  the  needle  is  attracted  by  the  north  pole,  the  un- 
marked end  is  repelled  by  an  equal  force,  the  two  equal 
and  opposite  forces  neutralizing  each  other. 

When  the  pole  of  an  ordinary  magnet  is  brought  to 
act  upon  the  swimming  needle,  the  latter  is  attracted — the 
reason  being  that  the  attracted  end  of  the  needle  being 
nearer  to  the  pole  of  the  magnet  than  the  repelled  end, 
the  force  of  attraction  is  the  more  powerful  of  the  two. 
In  the  case  of  the  earth,  its  pole  is  so  distant  that  the 
length  of  the  needle  is  practically  zero.  In  like  manner, 
when  a  piece  of  iron  is  presented  to  a  magnet,  the  nearer 
parts  are  attracted,  while  the  more  distant  parts  are  re- 
pelled; and  because  the  attracted  portions  are  nearer  to 
the  magnet  than  the  repelled  ones,  we  have  a  balance  in 
favor  of  attraction.  Here  then  is  the  special  characteris- 
tic of  the  magnetic  force,  which  distinguishes  it  from  that 
of  gravitation.  The  latter  is  a  simple  unpolar  force,  while 
the  former  is  duplex  or  polar.  Were  gravitation  like 
magnetism,  a  stone  would  no  more  fall  to  the  ground 
than  a  piece  of  iron  toward  the  north  magnetic  pole:  and 
thus,  however  rich  in  consequences  the  supposition  of 
Kepler  and  others  may  have  been,  it  is  clear  that  a  force 


340  FRAGMENTS    OF  SCIENCE 

like  that  of  magnetism  would  not  be  able  to  transact  the 
business  of  the  universe. 

The  object  of  this  discourse  is  to  inquire  whether  the 
force  of  diamagnetism,  which  manifests  itself  as  a  repul- 
sion of  certain  bodies  by  the  poles  of  a  magnet,  is  to  be 
ranged  as  a  polar  force,  beside  that  of  magnetism;  or  as 
an  unpolar  force,  beside  that  of  gravitation.  When  a 
cylinder  of  soft  iron  is  placed  within  a  wire  helix,  and 
surrounded  by  an  electric  current,  the  antithesis  of  its 
two  ends,  or,  in  other  words,  its  polar  excitation,  is  at 
once  manifested  by  its  action  upon  a  magnetic  needle; 
and  it  may  be  asked  why  a  cylinder  of  bismuth  may  not 
be  substituted  for  the  cylinder  of  iron,  and  its  state  simi- 
larly examined.  The  reason  is,  that  the  excitement  of  the 
bismuth  is  so  feeble  that  it  would  be  quite  masked  by 
that  of  the  helix  in  which  it  is  enclosed;  and  the  problem 
that  now  meets  us  is,  so  to  excite  a  diamagnetic  body  that 
the  pure  action  of  the  body  upon  a  magnetic  needle  may 
be  observed,  unmixed  with  the  action  of  the  body  used 
to  excite'  the  diamagnetic. 

How  this  has  been  effected  may  be  illustrated  in  the 
following  manner:  When  through  an  upright  helix  of 
covered  copper  wire  a  voltaic  current  is  sent,  the  top 
of  the  helix  attracts,  wbile  its  bottom  repels,  the  same 
pole  of  a  magnetic  needle;  its  central  point,  on  the  con- 
trary, is  neutral,  and  exhibits  neither  attraction  nor  repul- 
sion. Such  a  helix  is  caused  to  stand  between  the  two 
poles  n' s'  of  an  astatic  system.'  The  two  magnets  s  n' 
and  s'  N  are  united  by  a  rigid  cross  piece  at  their  centres, 

^  The  reversal  of  the  poles  of  the  two  magnets,  which  were  of  the  same 
strength,  completely  annulled  the  action  of  the  earth  as  a  magnet. 


PARAMAGNETIC    AND    VIAMAONETIC   FORCES       341 

and  are  suspended  from  the  point  a,  so  that  both  magnets 
swing  in  the  same  horizontal  plane.  It  is  so  arranged 
that  the  poles  n'  s'  are  opposite  to  the  central  or  neutral 
point  cf  the  helix,  so  that  when  a  current  is  sent  through 
the  latter,  the  magnets,  as  before  explained,  are  unaffected. 
Hare  then  we  have  an  excited  helix  which  itself  has  no 
action  upon  the  magnets,  and  we  are  thus  enabled  to  ex- 
amine the  action  of  a  body  placed  within  the  helix  and 
excited  by  it,  undisturbed  by  the  influence  of  the  latter. 
The  helix  being  12  inches  high,  a  cylinder  of  soft  iron 
6  inches  long,  suspended  from  a  string  and  passing  over  a 
pulley,  can  be  raised  or  lowered  within  the  helix.  When 
it  is  so  far  sunk  that  its  lower  end  rests  upon  the  table, 


NC 


Fig.  10. 


the  upper  end  finds  itself  between  the  poles  n'  s'  of  the 
astatic  system.  The  iron  cylinder  is  thus  converted  into  a 
strong  magnet,  attracting  one  of  the  poles,  and  repelling 
the  other,  and  consequently,  deflecting  the  entire  astatic 
system.  When  the  cylinder  is  raised  so  that  the  upper 
end  is  at  the  level  of  the  top  of  the  helix,  its  lower  end 
comes  between  the  poles  n'  s';  and  a  deflection  opposed  in 
direction  to  the  former  one  is  the  immediate  consequence. 
To  render  these  deflections  more  easily  visible,  a  mirror 
m  is  attached  to  the  system  of  magnets;  a  beam  of  light 
thrown  upon  the  mirror  being  reflected  and  projected  as  a 
bright  disk  against  the  wall.  The  distance  of  this  image 
from  the  mirror  being  considerable,  and  its  angular  mo- 


o4:2  FRAGMENTS   OF  SCIENCE 

tion  double  tliat  of  the  latter,  a  very  slight  motion  of 
the  magnet  is  sufficient  to  produce  a  displacement  of  the 
image  through  several  yards. 

This  then  is  the  principle  of  the  beautiful  apparatus* 
by  which  the  investigation  was  conducted.  It  is  manifest 
that  if  a  second  helix  be  placed  between  the  poles  s  N 
with  a  cylinder  within  it,  the  action  upon  the  astatic  mag- 
net may  be  exalted.  This  was  the  arrangement  made  use 
of  in  the  actual  inquiry.  Thus  to  intensify  the  feeble  ac- 
tion, which  it  is  here  our  object  to  seek,  we  have  in  the 
first  place  neutralized  the  action  of  the  earth  upon  the 
magnets,  by  placing  them  astatically.  Secondly,  by  mak- 
ing use  of  two  cylinders,  and  permitting  them  to  act 
simultaneously  on  the  four  poles  of  the  magnets,  we  have 
rendered  the  deflecting  force  four  times  what  it  would  be, 
if  only  a  single  pole  were  used.  Finally,  the  whole  ap- 
paratus was  enclosed  in  a  suitable  case  which  protected 
the  magnets  from  air-currents,  and  the  deflections  were 
read  off  through  a  glass  plate  in  the  case,  by  means  of 
a  telescope  and  scale  placed  at  a  considerable  distance 
from   the   instrument. 

A  pair  of  bismuth  cylinders  was  first  examined.  Send- 
ing a  current  through  the  helices,  and  observing  that  the 
magnets  swung  perfectly  free,  it  was  first  arranged  that 
the  bismuth  cylinders  within  the  helices  had  their  central 
or  neutral  points  opposite  to  the  poles  of  the  magnets. 
All  being  at  rest,  the  number  on  the  scale  marked  by  the 
cross  wire  of  the  telescope  was  572.  The  cylinders  were 
then  moved,  one  up,  the  other  down,  so  that  two  of  their 
ends  were  brought  to  bear  simultaneously  upon  the  mag- 

*  Devised  by  Professor  W.  Weber,  and  constructed  by  M.  Leyser,  of  Leipzig. 


PARAMAGNETIC  AND  DIAMAONETIC  FORCES       343 

netic  poles:  the  magnet  moved  promptly,  and  after  some 
oscillations'  came  to  rest  at  the  number  612;  thus  moving 
from  a  smaller  to  a  larger  number.  The  other  two  ends 
of  the  bars  were  next  brought  to  bear  upon  the  magnet: 
a  prompt  deflection  was  the  consequence,  and  the  final 
position  of  equilibrium  was  526:  the  movement  being  from 
a  larger  to  a  smaller  number.  We  thus  observe  a  mani- 
fest polar  action  of  the  bismuth  cylinders  upon  the  mag- 
net; one  pair  of  ends  deflecting  it  in  one  direction,  and 
the  other  pair  deflecting  it  in  the  opposite  direction. 

Substituting  for  the  cylinders  of  bismuth  thin  cylinders 
of  iron,  of  magnetic  slate,  of  sulphate  of  iron,  carbonate 
of  iron,  protochloride  of  iron,  red  ferrocyanide  of  potas- 
sium, and  other  magnetic  bodies,  it  was  found  that  when 
the  position  of  the  magnetic  cylinders  was  the  same  as 
that  of  the  cylinders  of  bismuth,  the  deflection  produced 
by  the  former  was  always  opposed  in  direction  to  that 
produced  by  the  latter;  and  hence  the  disposition  of  the 
force  in  the  diamagnetic  body  must  have  been  precisely 
antithetical  to  its  disposition  in  the  magnetic  ones. 

But  it  will  be  urged,  and  indeed  has  been  urged,  against 
this  inference,  that  the  deflection  produced  by  the  bismuth 
cylinders  may  be  due  to  induced  currents  excited  in  the 
metal  by  its  motion  within  the  helices.  In  reply  to  this 
objection,  it  may  be  stated,  in  the  first  place,  that  the  de- 
flection is  permanent,  and  cannot  therefore  be  due  to  in- 
duced currents,  which  are  only  of  momentary  duration. 
It  has  also  been  urged  that  such  experiments  ought  to  be 
made  with  other  metals,  and  with  better  conductors  than 
bismuth;    for  if   due  to  currents  of  induction,   the   better 

*  To  lessen  these  a  copper  damper  was  made  use  of. 


344  FRAGMENTS   OF  SCIENCE 

the  conductor  the  more  exalted  will  be  the  effect.     This 
requirement  was  complied  with. 

Cylinders  of  antimony  were  substituted  for  those  of 
bismuth.  This  metal  is  a  better  conductor  of  electricity, 
but  less  strongly  diamagnetic  than  bismuth.  If  therefore 
the  action  referred  to  be  due  to  induced  currents,  we  ought 
to  have  it  greater  in  the  case  of  antimony  than  with  bis- 
muth; but  if  it  springs  from  a  true  diamagnetic  polarity, 
the  action  of  the  bismuth  ought  to  exceed  that  of  the  anti- 
mony. Experiment  proves  this  to  be  the  case.  Hence 
the  deflection  produced  by  these  metals  is  due  to  their 
diamagnetic,  and  not  to  their  conductive  capacity.  Cop- 
per cylinders  were  next  examined:  here  we  have  a  metal 
which  conducts  electricity  fifty  times  better  than  bismuth, 
but  its  diamagnetic  power  is  nearly  null;  if  the  effects  be 
due  to  induced  currents  we  ought  to  have  them  here  in 
an  enormously  exaggerated  degree,  but  no  sensible  deflec- 
tion was  produced  by  the  two  cylinders  of  copper. 

It  has  also  been  proposed  by  the  opponents  of  diamag- 
-netic  polarity  to  coat  fragments  of  bismuth  with  some  in- 
sulating substance,  so  as  to  render  the  formation  of  induced 
currents  impossible,  and  to  test  the  question  with  cylin- 
ders of  these  fragments.  This  requirement  was  also  ful- 
filled. It  is  only  necessary  to  reduce  the  bismuth  to  pow- 
der and  expose  it  for  a  short  time  to  the  air  to  cause  the 
particles  to  become  so  far  oxidized  as  to  render  them  per- 
fectly insulating.  The  insulating  power  of  the  powder  was 
exhibited  experimentally;  nevertheless,  this  powder,  en- 
closed in  glass  tubes,  exhibited  an  action  scarcely  less 
powerful  than  that  of  the  massive  bismuth  cylinders. 

But  the  most  rigid  proof,  a  proof  admitted  to  be  con- 
clusive by  those  who  have  denied  the  antithesis  of  mag- 


PARAMAGNETIC  AND  DIAMAGNETIC  FORCES       oio 

netism  and  diamagnetism,  remains  to  be  stated.  Prisms 
of  the  same  heavy  glass  as  that  with  which  the  diamag- 
netic  force  was  discovered  were  substituted  for  the  metal- 
lic cylinders,  and  their  action  upon  the  magnet  was  proved 
to  be  precisely  the  same  in  kind  as  that  of  the  cylinders 
of  bismuth.  The  inquiry  was  also  extended  to  other  in- 
sulators: to  phosphorus,  sulphur,  nitre,  calcareous  spar, 
statuary  marble,  with  the  same  invariable  result:  each  of 
these  substances  was  proved  to  be  polar,  the  disposition 
of  the  force  being  the  same  as  that  of  bismuth  and  the 
reverse  of  that  of  iron.  When  a  bar  of  iron  is  set  erect, 
its  lower  end  is  known  to  be  a  north  pole,  and  its  upper 
end  a  south  pole,  in  virtue  of  the  earth's  induction.  A 
marble  statue,  on  the  contrary,  has  its  feet  a  south  pole, 
and  its  head  a  north  pole,  and  there  is  no  doubt  that  the 
same  remark  applies  to  its  living  archetype;  each  man 
walking  over  the  earth's  surface  is  a  true  diamagnet, 
with  its  poles  the  reverse  of  those  of  a  mass  of  magnetic 
matter  of  the  same  shape  and  position. 

An  experiment  of  practical  value,  as  affording  a  ready 
estimate  of  the  different  conductive  powers  of  two  metals 
for  electricity,  was  exhibited  in  the  lecture,  for  the  pur- 
pose of  proving  experimentally  some  of  the  statements 
made  in  reference  to  this  subject.  A  cube  of  bismuth  was 
suspended  by  a  twisted  string  between  the  two  poles  of 
an  electro-magnet.  The  cube  was  attached  by  a  short 
copper  wire  to  a  little  square  pyramid,  the  base  of  which 
was  horizontal,  and  its  sides  formed  of  four  small  trian- 
gular pieces  of  looking-glass.  A  beam  of  light  was  suf- 
fered to  fall  upon  this  reflector,  and  as  the  reflector  fol- 
lowed the  motion  of  the  cube  the  images  cast  from  its 
sides  followed  each   other  in  succession,   each  describing 


346  FRAGMENTS    OF   SCIENCE 

a  circle  about  thirty  feet  in  diameter.  As  t"he  velocity  of 
rotation  augmented,  these  images  blended  into  a  continu- 
ous ring  of  light.  At  a  particular  instant  the  electro- 
magnet was  excited,  currents  were  evolved  in  the  rotating 
cube,  and  the  strength  of  these  currents,  which  increases 
with  the  conductivity  of  the  cube  for  electricity,  was 
practically  estimated  by  the  time  required  to  bring  the 
cube  and  its  associated  mirrors  to  a  state  of  rest.  With 
bismuth  this  time  amounted  to  a  score  of  seconds  or  more: 
a  cube  of  copper,  on  the  contrary,  was  struck  almost  in- 
stantly motionless  when  the  circuit  was  established. 


XIV 

PHYSICAL   BASIS   OF   SOLAR   CHEMISTRY* 

OMITTING-  all  preface,  attention  was  first  drawn  to 
an  experimental  arrangement  intended  to  prove 
that  gaseous  bodies  radiate  lieat  in  different  de- 
grees. Near  a  double  screen  of  polished  tin  was  placed 
an  ordinary  ring  gas-burner,  and  on  this  was  placed  a  hot 
copper  ball,  from  which  a  column  of  heated  air  ascended. 
Behind  the  screen,  but  so  situated  that  no  ray  from  the 
ball  could  reach  the  instrument,  was  an  excellent  thermo- 
electric pile,  connected  by  wires  with  a  very  delicate  gal- 
vanometer. The  pile  was  known  to  be  an  instrument 
whereby  heat  is  applied  to  the  generation  -x>i  electric  cur- 
reiits;  the  strength  of  the  current  being  an  accurate  meas- 
ure of  the  quantity  of  the  heat.  As  long  as  both  faces 
of  the  pile  are  at  the  same  temperature,  no  current  is  pro- 
duced; but  the  slightest  difference  in  the  temperature  of 
the  two  faces  at  once  declares  itself  by  the  production  of  a 
current,  which,  when  carried  through  the  galvanometer, 
indicates  by  the  deflection  of  the  needle  both  its  strength 
and  its  direction. 

The  two  faces  of  the  pile  were  in  the  first  instance 
brought  to  the  same  temperature;  the  equilibrium  being 
shown  by  the  needle  of  the  galvanometer  standing  at  zero. 

'  From  a  discourse  delivered  at  the  Royal  Institution  of  Great  Britain,  June 
1,  1861. 

(347) 


348  FRAGMENTS    OF   SCIENCE 

Tlie  rays  emitted  by  the  current  of  hot  air  already  referred 
to  were  permitted  to  fall  upon  one  of  the  faces  of  the  pile; 
and  an  extremely  slight  movement  of  the  needle  showed 
that  the  radiation  from  the  hot  air,  though  sensible,  was 
extremely  feeble.  Connected  with  the  ring- burner  was  a 
holder  containing  oxygen  gas;  and  by  turning  a  cock,  a 
stream  of  this  gas  was  permitted  to  issue  from  the  burner, 
strike  the  copper  ball,  and  ascend  in  a  heated  column  in 
front  of  the  pile.  The  result  was,  that  oxygen  showed 
itself,  as  a,  radiator  of  heat,  to  be  quite  as  feeble  as  at- 
mospheric air. 

A  second  holder  containing  olefiant  gas  was  then  con- 
nected with  the  ring-burner.  Oxygen  and  air  had  already 
flowed  over  the  ball  and  cooled  it  in  some  degree.  Hence 
the  olefiant  gas  labored  under  a  disadvantage.  But  on 
permitting  the  gas  to  rise  from  the  ball,  it  cast  an  amount 
of  heat  against  the  adjacent  face  of  the  pile  sufficient  to 
impel  the  needle  of  the  galvanometer  almost  to  90°.  This 
experiment  proved  the  vast  difference  between  two  equally 
invisible  gases  with  regard  to  their  power  of  emitting  ra- 
diant heat. 

The  converse  experiment  was  now  performed.  The 
thermo-electric  pile  was  removed  and  placed  between  two 
cubes  filled  with  water  kept  in  a  state  of  constant  ebulli- 
tion; and  it  was  so  arranged  that  the  quantities  of  heat 
falling  from  the  cubes  on  the  opposite  faces  of  the  pile 
were  exactly  equal,  thus  neutralizing  each  other.  The 
needle  of  the  galvanometer  being  at  zero,  a  sheet  of  oxy- 
gen gas  was  caused  to  issue  from  a  slit  between  one  of  the 
cubes  and  the  adjacent  face  of  the  pile.  If  this  sheet  of 
gas  possessed  any  sensible  power  of  intercepting  the  ther- 
mal rays  from  the  cube,   one  face   of   the   pile  being  de- 


PHYSICAL    BASTS    OF   SOLAR    CHEMISTRY  349 

prived  of  the  heat  thus  intercepted,  a  difference  of  tem- 
perature between  its  two  faces  would  instantly  set  in,  and 
the  result  would  be  declared  by  the  galvanometer.  The 
quantity  absorbed  by  the  oxygen  under  those  circum- 
stances was  too  feeble  to  affect  the  galvanometer;  the  gas, 
in  fact,  proved  perfectly  transparent  to  the  rays  of  heat. 
It  had  but  a  feeble  power  of  radiation:  it  had  an  equally 
feeble  power  of  absorption. 

The  pile  remaining  in  its  position,  a  sheet  of  oleiiant 
gas  was  caused  to  issue  from  the  same  slit  as  that  through 
which  the  oxygen  had  passed.  No  one  present  could  see 
the  gas;  it  was  quite  invisible,  the  light  went  through  it 
as  freely  as  through  oxygen  or  air;  but  its  effect  upon  the 
thermal  rays  emanating  from  the  cube  was  what  might  be 
expected  from  a  sheet  of  metal.  A  quantity  so  large  was 
cut  off  that  the  needle  of  the  galvanometer,  promptly  quit- 
ting the  zero  line,  moved  with  energy  to  its  stops.  Thus 
the  olefiaut  gas,  so  light  and  clear  and  pervious  to  lumi- 
nous rays,  was  proved  to  be  a  most  potent  destroyer  of  the 
rays  emanating  from  an  obscure  source.  The  reciprocity 
of  action  established  in  the  case  of  oxygen  comes  out  here; 
the  good  radiator  is  found  by  this  experiment  to  be  the 
good  absorber. 

This  result,  now  exhibited  before  a  public  audience  for 
the  first  time,  was  typical  of  what  had  been  obtained  with 
gases  generally.  Going  through  the  entire  list  of  gases 
and  vapors  in  this  way,  we  find  radiation  and  absorption 
to  be  as  rigidly  associated  as  positive  and  negative  in  elec- 
tricity, or  as  north  and  south  polarity  in  magnetism.  So 
that  if  we  make  the  number  which  expresses  the  absorp- 
tive power  the  numerator  of  a  fraction,  and  that  which 
expresses   its  radiative  power  the  denominator,  the  result 


850  FRAGMENTS    OF  SCIENCE 

would  be,  that  on  account  of  the  numerator  and  denom- 
inator varying  in  the  same  proportion,  the  value  of  that 
fraction  would  always  remain  the  same,  whatever  might 
be  the  gas  or  vapor  experimented  with. 

But  why  should  this  reciprocity  exist?  What  is  the 
meaning  of  absorption  ?  what  is  the  meaning  of  radiation  ? 
When  you  cast  a  stone  into  still  water,  rings  of  waves 
surround  the  place  where  it  falls;  motion  is  radiated  on 
all  sides  from  the  centre  of  disturbance.  When  a  ham- 
mer strikes  a  bell,  the  latter  vibrates;  and  sound,  which 
is  nothing  more  than  an  undulatory  motion  of  the  air,  is 
radiated  in  all  directions.  Modern  philosophy  reduces 
light  and  heat  to  the  same  mechanical  category.  A  lumi- 
nous body  is  one  with  its  atoms  in  a  state  of  vibration; 
a  hot  body  is  one  with  its  atoms  also  vibrating,  but  at  a 
rate  which  is  incompetent  to  excite  the  sense  of  vision; 
and,  as  a  sounding  body  has  the  air  around  it,  through 
which  it  propagates  its  vibrations,  so  also  the  luminous  or 
heated  body  has  a  medium,  called  ether,  which  accepts 
its  motions  and  carries  them  forward  with  inconceivable 
velocity.  Eadiation,  then,  as  regards  both  light  and  heat, 
is  the  transference  of  motion  from  the  vibrating  body  to 
the  ether  in  which  it  swings:  and,  as  in  the  case  of 
sound,  the  motion  imparted  to  the  air  is  soon  transferred 
to  surrounding  objects,  against  which  the  aerial  undula- 
tions strike,  the  sound  being,  in  technical  language,  ah- 
sorbed;  so  also  with  regard  to  light  and  heat,  absorption 
consists  in  the  transference  of  motion  from  the  agitated 
ether  to  the  molecules  of  the  absorbing  body. 

The  simple  atoms  are  found  to  be  bad  radiators;  the 
compound  atoms  good  ones:  and  the  higher  the  degree  of 
complexity  in  the  atomic  grouping,  the  more  potent,  as  a 


PHYSICAL    BASIS    OF   SOLAR    CHEMISTRY  351 

general  rule,  is  the  radiation  and  absorption.  Let  ns  get 
definite  ideas  here,  however  gross,  and  purify  them  after- 
ward by  the  process  of  abstraction.  Imagine  our  simple 
atoms  swinging  like  single  spheres  in  the  ether;  they 
cannot  create  the  swell  which  a  group  of  them  united  to 
form  a  system  can  produce.  An  oar  runs  freely  edgewise 
through  the  water,  and  imparts  far  less  of  its  motion  to 
the  water  than  when  its  broad,  flat  side  is  brought  to  bear 
upon  it  In  our  present  language  the  oar,  broad  side  ver- 
tical, is  a  good  radiator;  broad  side  horizontal,  it  is  a  bad 
radiator.  Conversely  the  waves  of  water,  impinging  upon 
the  flat  face  of  the  oar- blade,  will  impart  a  greater  amount 
of  motion  to  it  than  when  impinging  upon  the  edge.  In 
the  position  in  which  the  oar  radiates  well,  it  also  absorbs 
well.  Simple  atoms  glide  through  the  ether  without  much 
resistance;  compound  ones  encounter  resistance,  and  hence 
yield  up  more  speedily  their  motion  to  the  ether.  Mix 
oxygen  and  nitrogen  mechanically,  they  absorb  and  radi- 
ate a  certain  amount  of  heat.  Cause  these  gases  to  com- 
bine chemically  and  form  nitrous  oxide,  both  the  absorp- 
tion and  radiation  are  thereby  augmented  hundreds  of 
times! 

In  this  way  we  look  with  the  telescope  of  the  intellect 
into  atomic  systems,  and  obtain  a  conception  of  processes 
which  the  eye  of  sense  can  never  reach.  But  gases  and 
vapors  possess  a  power  of  choice  as  to  the  rays  which 
they  absorb.  They  single  out  certain  groups  of  rays  for 
destruction,  and  allow  other  groups  to  pass  unharmed. 
This  is  best  illustrated  by  a  famous  experiment  of  Sir 
DaTid  Brewster's,  modified  to  suit  present  requirements. 
Into  a  glass  cylinder,  with  its  ends  stopped  by  disks  of 
plate-glass,  a  small  quantity  of  nitrous  acid  gas  is  intro- 


352  FRAGMENTS   OF  SCIENCE 

duced;  the  presence  of  the  gas  being  indicated  by  its  rich 
brown  color.  The  beam  from  an  electric  lamp  being  sent 
through  two  prisms  of  bisulphide  of  carbon,  a  spectrum 
seven  feet  long  and  eighteen  inches  wide  is  cast  upon  the 
screen.  Introducing  the  cylinder  containing  the  nitrous 
acid  into  the  path  of  the  beam  as  it  issues  from  the  lamp, 
the  splendid  and  continuous  spectrum  becomes  instantly 
furrowed  by  numerous  dark  bands,  the  rays  answering  to 
which  are  intercepted  by  the  nitric  gas,  while  the  light 
which  falls  upon  the  intervening  spaces  is  permitted  to 
pass  with  comparative  impunity. 

Here  also  the  principle  of  reciprocity,  as  regards  radi- 
ation and  absorption,  holds  good;  and  could  we,  without 
otherwise  altering  its  physical  character,  render  that  ni- 
trous gas  luminous,  we  should  find  that  the  very  rays 
which  it  absorbs  are  precisely  those  which  it  would  emit. 
When  atmospheric  air  and  other  gases  are  brought  to  a 
state  of  intense  incandescence  by  the  passage  of  an  elec- 
tric spark,  the  spectra  which  we  obtain  from  them  consist 
of  a  series  of  bright  bands.  But  such  spectra  are  pro- 
duced with  the  greatest  brilliancy  when,  instead  of  ordi- 
nary gases,  we  make  use  of  metals  heated  so  highly  as  to 
volatilize  them.  This  is  easily  done  by  the  voltaic  cur- 
rent. A  capsule  of  carbon  filled  with  mercury,  which 
formed  the  positive  electrode  of  the  electric  lamp,  has  a 
carbon  point  brought  down  upon  it.  On  separating  the 
one  from  the  other,  a  brilliant  arc  containing  the  mercury 
in  a  volatilized  condition  passes  between  them.  The  spec- 
trum of  this  arc  is  not  continuous  like  that  of  the  solid 
carbon  points,  but  consists  of  a  series  of  vivid  bands, 
each  corresponding  in  color  to  that  particular  portion  of 
the  spectrum  to  which  its  rays  belong.     Copper  gives  its 


PHYSICAL   BASIS    OF   SOLAR    CHEMISTRY  853 

system  of  bands;  zinc  gives  its  system;  and  brass,  which 
is  an  alloy  of  copper  and  zinc,  gives  a  spectrum  made  up 
of  the  bands  belonging  to  both  metals. 

Not  only,  however,  when  metals  are  united  like  zinc 
and  copper  to  form  an  alloy  is  it  possible  to  obtain  the 
bands  which  belong  to  them.  No  matter  how  we  may  dis- 
guise the  metal — allowing  it  to  unite  with  oxygen  to  form 
an  oxide,  and  this  again  with  an  acid  to  form  a  salt;  if 
the  heat  applied  be  sufficiently  intense,  the  bands  belong- 
ing to  the  metal  reveal  themselves  with  perfect  definition. 
Into  holes  drilled  in  a  cylinder  of  retort  carbon,  pure  cul- 
inary salt  is  introduced.  When  the  carbon  is  made  the 
positive  electrode  of  the  lamp,  the  resultant  spectrum 
shows  the  brilliant  yellow  lines  of  the  metal  sodium. 
Similar  experiments  made  with  the  chlorides  of  stron- 
tium, calcium,  lithium,^  and  other  metals,  give  the  bands 
due  to  the  respective  metals.  When  difierent  salts  are 
mixed  together,  and  rammed  into  holes  in  the  carbon,  a 
spectrum  is  obtained  which  contains  the  bands  of  them  all. 

The  position  of  these  bright  bands  never  varies,  and 
each  metal  has  its  own  system.  Hence  the  competent 
observer  can  infer  from  the  bands  of  the  spectrum  the 
metals  which  produce  it.  It  is  a  language  addressed  to 
the  eye  instead  of  the  ear;  and  the  certainty  would  not 
be  augmented  if  each  metal  possessed  the  power  of  audi- 
bly calling  out,  "I  am  here!"  Nor  is  this  language  af- 
fected by  distance.     If  we  find  that  the  sun  or  the  stars 

^  The  vividness  of  the  colors  of  the  lithium  spectrum  is  extraordinary ;  the 
spectmm,  moreover,  contained  a  blue  band  of  indescribable  splendor.  It  was 
thought  by  many,  diiring  the  discourse,  that  I  hsui  mistaken  strontium  for 
lithium,  as  this  blue  band  had  never  before  been  seen.  I  have  obtained  it 
many  times  since;  and  my  friend  Dr.  Miller,  having  kindly  analyzed  the 
substance  made  use  of,  pronounces  it  pure  chloride  of  lithium. — J.  T. 


854  FRAGMENTS   OF  SCIENCE 

give  us  the  bands  of  our  terrestrial  metals,  it  is  a  decla- 
ration on  the  part  of  these  orbs  that  such  metals  enter 
into  their  composition.  Does  the  sun  give  us  any  such 
intimation?  Does  the  solar  spectrum  exhibit  bright  lines 
"which  we  might  compare  with  those  produced  by  our  ter- 
restrial metals,  and  prove  either  their  identity  or  differ- 
ence? No.  The  solar  spectrum,  when  closely  examined, 
gives  us  a  multitude  of  fine  dark  lines  instead  of  bright 
ones.  They  were  first  noticed  by  Dr.  Wollaston,  but  were 
multiplied  and  investigated  with  profound  skill  by  Fraun- 
hofer,  and  named,  after  him,  Fraunhofer's  lines.  They 
had  been  long  a  standing  puzzle  to  philosophers.  The 
bright  lines  yielded  by  metallic  vapors  had  been  also 
known  to  us  for  years;  but  the  connection  between  both 
classes  of  phenomena  was  wholly  unknown,  until  Kirch- 
hoff,  with  admirable  acuteness,  revealed  the  secret,  and 
placed  it  at  the  same  time  in  our  power  to  chemically 
analyze  the  sun. 

We  have  now  some  difficult  work  before  us.  Hitherto 
we  have  been  delighted  by  objects  which  addressed  them- 
selves as  much  to  our  aesthetic  taste  as  to  our  scientific 
raculty;  we  have  ridden  pleasantly  to  the  base  of  the  final 
cone  of  Etna,  and  must  now  dismount  and  march  through 
ashes  and  lava,  if  we  would  enjoy  the  prospect  from  the 
summit.  Our  problem  is  to  connect  the  dark  lines  of 
Fraunhofer  with  the  bright  ones  of  the  metals.  The  white 
beam  of  the  lamp  is  refracted  in  passing  through  our  two 
prisms,  but  its  different  components  are  refracted  in  differ- 
ent degrees,  and  thus  its  colors  are  drawn  apart.  Now, 
the  color  depends  solely  upon  the  rate  of  oscillation  of  the 
atoms  of  the  luminous  body;  red  light  being  produced  by 
one  rate,  blue  light  by  a  much  quicker  rate,  and  the  col- 


PHYSICAL   BASIS   OF  SOLAR    CHEMISTRY  855 

ors  between  red  and  blue  by  the  intermediate  rates.  The 
solid  incandescent  coal-points  give  us  a  continuous  spec- 
trum; or,  in  other  words,  they  emit  rays  of  all  possible 
periods  between  the  two  extremes  of  the  spectrum.  Color, 
as  many  of  you  know,  is  to  light  what  pitch  is  to  sound. 
When  a  violin -player  presses  his  finger  on  a  string  he 
makes  it  shorter  and  tighter,  and  thus,  causing  it  to 
vibrate  more  speedily,  heightens  the  pitch.  Imagine  such 
a  player  to  move  his  fingers  slowly  along  the  string,  short- 
ening it  gradually  as  he  draws  his  bow,  the  note  would 
rise  in  pitch  by  a  regular  gradation;  there  would  be  no 
gap  intervening  between  note  and  note.  Here  we  have 
the  analogue  to  the  continuous  spectrum,  whose  colors  in- 
sensibly blend  together  without  gap  or  interruption,  from 
the  red  of  the  lowest  pitch  to  the  violet  of  the  highest. 
But  suppose  the  player,  instead  of  gradually  shortening 
his  string,  to  press  his  finger  on  a  certain  point,  and  to 
sound  the  corresponding  note;  then  to  pass  on  to  another 
point  more  or  less  distant,  and  sound  its  note ;  then  to  an 
other,  and  so  on,  thus  sounding  particular  notes  separated 
from  each  other  by  gaps  which  correspond  to  the  intervals 
of  the  string  passed  over;  we  should  then  have  the  exact 
analogue  of  a  spectrum  composed  of  separate  bright  bands 
with  intervals  of  darkness  between  them.  But  this, 
though  a  perfectly  true  and  intelligible  analogy,  is  not 
sufiicient  for  our  purpose;  we  must  look  with  the  mind's 
eye  at  the  oscillating  atoms  of  the  volatilized  metal.  Fig- 
ure these  atoms  as  connected  together  by  springs  of  a 
certain  tension,  which,  if  the  atoms  are  squeezed  together, 
push  them  again  asunder,  and  if  the  atoms  are  drawn 
apart,  pull  them  again  together,  causing  them,  before  com- 
ing to  rest,  to  quiver  for  a  certain  time  at  a  certain  defi- 


856  FRAGMENTS    OF   SCIENCE 

nite  rate  determined  by  the  strength  of  the  spring.  Now, 
the  volatilized  metal  which  gives  ns  one  bright  band  is 
to  be  figured  as  having  its  atoms  united  by  springs  all  of 
the  same  tension;  its  vibrations  are  all  of  one  kind.  The 
metal  which  gives  us  two  bands  may  be  figured  as  having 
some  of  its  atoms  united  by  springs  of  one  tension,  and 
others  by  springs  of  a  different  tension.  Its  vibrations 
are  of  two  distinct  kinds;  so  also  when  we  have  three 
or  more  bands  we  are  to  figure  as  many  distinct  sets  of 
springs,  each  capable  of  vibrating  in  its  own  particular 
time  and  at  a  different  rate  from  the  others.  If  we  seize 
this  idea  definitely,  we  shall  have  no  difficulty  in  dropping 
the  metaphor  of  springs,  and  substituting  for  it  mentally 
the  forces  by  which  the  atoms  act  upon  each  other.  Hav- 
ing thus  far  cleared  our  way,  let  us  make  another  effort 
to  advance. 

A  heavy  ivory  ball  is  here  suspended  from  a  string. 
I  blow  against  this  ball ;  a  single  puff  of  my  breath  moves 
it  a  little  way  from  its  position  of  rest;  it  swings  back 
toward  me,  and  when  it  reaches  the  limit  of  its  swing  I 
puff  again.  It  now  swings '^  further ;  and  thus  by  timing 
the  puffs  I  can  so  accumulate  their  action  as  to  produce 
oscillations  of  large  amplitude.  The  ivory  ball  here  has 
absorbed  the  motion  which  my  breath  communicated  to 
the  air.  I  now  bring  the  ball  to  rest.  Suppose,  instead 
of  the  breath,  a  wave  of  air  to  strike  against  it,  and  that 
this  wave  is  followed  by  a  series  of  others  which  succeed 
each  other  exactly  m  the  same  intervals  as  my  puffs;  it 
is  obvious  that  these  waves  would  communicate  their  mo- 
tion to  the  ball  and  cause  it  to  swing  as  the  puffs  did. 
And  it  is  equally  manifest  that  this  would  not  be  the  case 
if  the  impulses  of  the  waves  were  not  properly  timed;   for 


PHYSICAL   BASIS   OF  SOLAR    CHEMISTRY  857 

then  the  motion  imparted  to  the  pendulum  by  one  wave 
would  be  neutralized  by  another,  and  there  could  not  be 
the  accumulation  of  effect  obtained  when  the  periods  of 
the  waves  correspond  with  the  periods  of  the  pendulum. 
So  much  for  the  particular  impulses  absorbed  by  the  pen- 
dulum. But  if  such  a  pendulum  set  oscillating  in  air  could 
produce  waves  in  the  air,  it  is  evident  that  the  waves  it 
would  produce  would  be  of  the  same  period  as  those  whose 
motions  it  would  take  up  or  absorb  most  completely,  if 
they  struck  against  it. 

Perhaps  the  most  curious  effect  of  these  timed  impulses 
ever  described  was  that  observed  by  a  watchmaker,  named 
Ellicott,  in  the  year  1741.  He  left  two  clocks  leaning 
against  the  same  rail;  one  of  them,  which  we  may  call  A, 
was  set  going;  the  other,  B,  not.  Some  ytime  afterward 
he  found,  to  his  surprise,  that  B  was  ticking  also.  The 
pendulums  being  of  the  same  length,  the  shocks  imparted 
by  the  ticking  of  A  to  the  rail  against  which  both  clocks 
rested  were  propagated  to  B,  and  were  so  timed  as  to  set 
B  going.  Other  curious  effects  were  at  the  same  time  ob- 
served. When  the  pendulums  differed  from  each  other 
a  certain  amount,  A  set  B  going,  but  the  reaction  of  B 
stopped  A.  Then  B  set  A  going,  and  the  reaction  of  A 
stopped  B.  When  the  periods  of  oscillation  were  close  to 
each  other,  but  still  not  quite  alike,  the  clocks  mutually 
controlled  each  other,  and  by  a  kind  of  compromise  they 
ticked  in  perfect  unison. 

But  what  has  all  this  to  do  with  our  present  subject  ?  The 
varied  actions  of  the  universe  are  all  modes  of  motion; 
and  the  vibration  of  a  ray  claims  strict  brotherhood  with 
the  vibrations  of  our  pendulum.  Suppose  ethereal  waves 
striking  upon  atoms  which  oscillate  in  the  same  periods 


858  FRAGMEXTS    OF  SCIENCE 

as  the  waves,  the  motion  of  the  waves  will  be  absorbed 
by  the  atoms;  suppose  we  send  our  beam  of  white  light 
through  a  sodium  flame,  the  atoms  of  that  flame  will  be 
chiefly  affected  by  those  undulations  which  are  synchro- 
nous with  their  own  periods  of  vibration.  There  will  be 
on  the  part  of  those  particular  rays  a  transference  of  mo- 
tion from  the  agitated  ether  to  the  atoms  of  the  volatilized 
metal,  which,  as  already  defined,  is  absorption. 

The  experiment  justifying  this  conclusion  is  now  for 
the  first  time  to  be  made  before  a  public  audience.  I  pass 
a  beam  through  our  two  prisms,  and  the  spectrum  spreads 
its  colors  upon  the  screen.  Between  the  lamp  and  the 
prism  I  interpose  a  snap-dragon  light.  Alcohol  and  water 
are  here  mixed  with  common  salt,  and  the  metal  dish 
that  holds  them  is  heated  by  a  spirit-lamp.  The  vapor 
from  the  mixture  ignites  and  we  have  a  monochromatic 
flame.  Through  this  flame  the  beam  from  the  lamp  is 
now  passing;  and  observe  the  result  upon  the  spectrum. 
You  see  a  shady  band  cut  out  of  the  yellow — not  very- 
dark,  but  sufficiently  so  to  be  seen  by  everybody  present. 

But  let  me  exalt  this  effect.  Placing  in  front  of  the 
electric  lamp  the  intense  flame  of  a  large  Bunsen's  burner, 
a  platinum  capsule  containing  a  bit  of  sodium  less  than  a 
pea  in  magnitude  is  plunged  into  the  flame.  The  sodium 
soon  volatilizes  and  burns  with  brilliant  incandescence. 
The  beam  crosses  the  flame,  and  at  the  same  time  the. yel- 
low band  of  the  spectrum  is  clearly  and  sharply  cut  out, 
a  band  of  intense  darkness  occupying  its  place.  On  with- 
drawing the  sodium,  the  brilliant  yellow  of  the  spectrum 
takes  its  proper  place,  while  the  reintroduction  of  the 
flame  causes  the  band  to  reappear. 

Let    me    be    more    precise.     The    yellow   color   of   the 


PHYSICAL   BASIS    OF  SOLAR    CHEMISTRY  859 

spectrum  extends  over  a  sensible  space,  blending  on  one 
side  with  the  orange  and  on  the  other  with  the  green. 
The  term  "yellow  band"  is.  therefore  somewhat  indefinite. 
This  vagueness  may  be  entirely  removed.  By  dipping  the 
carbon-point  used  for  the  positive  electrode  into  a  solu- 
tion of  common  salt,  and  replacing  it  in  the  lamp,  the 
bright  yellow  band  produced  by  the  sodium  vapor  stands 
out  from  the  spectrum.  When  the  sodium  flame  is  caused 
to  act  upon  the  beam  it  is  that  particular  yellow  band  that 
is  obliterated,  an  intensely  black  streak  occupying  its 
place. 

An  additional  step  of  reasoning  leads  to  the  conclusion 
that  if,  instead  of  the  flame  of  sodium  alone,  we  were  to 
introduce  into  the  path  of  the  beam  a  flame  in  which  lith- 
ium, strontium,  magnesium,  calcium,  etc.,  are  in  a  state 
of  volatilization,  each  metallic  vapor  would  cut  out  a  sys- 
tem of  bands,  corresponding  exactly  in  position  with  the 
bright  bands  of  the  same  metallic  vapor.  The  light  of  our 
electric  lamp  shining  through  such  a  composite  flame 
would  give  us  a  spectrum  cut  up  by  dark  lines,  exactly 
as  the  solar  spectrum  is  cut  up  by  the  lines  of  Fraunhofer. 

Thus  by  the  combination  of  the  strictest  reasoning  with 
the  most  conclusive  experiment  we  reach  the  solution  of 
one  of  the  grandest  of  scientific  problems — the  constitu- 
tion of  the  sun.  The  sun  consists  of  a  nucleus  surrounded 
by  a  flaming  atmosphere.  The  light  of  the  nucleus  would 
give  us  a  continuous  spectrum,  like  that  of  our  common 
carbon-points;  but  having  to  pass  through  the  photo- 
sphere, as  our  beam  had  to  pass  through  the  flame,  those 
rays  of  the  nucleus  which  the  photosphere  can  itself  emit 
are  absorbed,  and  shaded  spaces,  corresponding  to  the 
particular  rays  absorbed,  occur  in  the  spectrum.     Abolish 


860  FRAGMENTS   OF   SCIENCE 

the  solar  nucleus,  and  we  should  have  a  spectrum  show- 
ing a  bright  line  in  the  place  of  every  dark  line  of  Fraun- 
hofer.  These  lines  are  therefore  not  absolutely  dark,  but 
dark  by  an  amount  corresponding  to  the  difference  be- 
tween the  light  of  the  nucleus  intercepted  by  the  photo- 
sphere, and  the  light  which  issues  from  the  latter. 

The  man  to  whom  we  owe  this  noble  generalization  is 
Kirchhoff,  Professor  of  Natural  Philosophy  in  the  Uni- 
versity of  Heidelberg;'  but,  like  every  other  great  discov- 
ery, it  is  compounded  of  various  elements.  Mr.  Talbot 
observed  the  bright  lines  in  the  spectra  of  colored  flames. 
Sixteen  years  ago  Dr.  Miller  gave  drawings  and  descrip- 
tions of  the  spectra  of  various  colored  flames.  Wheat- 
stone,  with  his  accustomed  ingenuity,  analyzed  the  light 
of  the  electric  spark,  and  showed  that  the  metals  between 
which  the  spark  passed  determined  the  bright  bands  in 
the  spectrum  of  the  spark.  Masson  published  a  prize  es- 
say on  these  bands;  Van  der  Willigen,  and  more  recently 
Pliicker,  have  given  us  beautiful  drawings  of  the  spectra, 
obtained  from  the  discharge  of  Ruhmkorff's  coil.  But 
none  of  these  distinguished  men  betrayed  the  least  knowl- 
edge of  the  connection  between  the  bright  bands  of  the 
metals  and  the  dark  lines  of  the  solar  spectrum.  The 
man  who  came  nearest  to  the  philosophy  of  the  subject 
was  Angostrom.  In  a  paper  translated  from  Poggendorff's 
"Annalen"  by  myself,  and  published  in  the  "Philosoph- 
ical Magazine"  for  1865,  he  indicates  that  the  rays  which 
a  body  absorbs  are  precisely  those  which  it  can  emit  when 
rendered  luminous.  In  another  place,  he  speaks  of  one 
of  his  spectra  giving  the  general  impression  of  a  reversal 


^  Now  Professor  in  the  University  of  Berlin. 


PHYSICAL   BASIS    OF  SOLAR    CHEMISTRY  861 

of  the  solar  spectrum.  Foucault,  Stokes,  and  Thomson, 
have  all  been  very  close  to  the  discovery;  and,  for  my 
own  part,  the  examination  of  the  radiation  and  absorption 
of  heat  by  gases  and  vapors,  some  of  the  results  of  which 
I  placed  before  you  at  the  commencement  of  this  dis- 
course,  would  have  led  me  in  1859  to  the  law  on  which 
all  Kirchhoff's  speculations  are  founded,  had  not  an  acci- 
dent withdrawn  me  from  the  investigation.  But  Kirch- 
hoff's  claims  are  unaffected  by  these  circumstances.  True, 
much  that  I  have  referred  to  formed  the  necessary  basis 
of  his  discovery;  so  did  the  laws  of  Kepler  furnish  to 
Newton  the  basis  of  the  theory  of  gravitation.  But  what 
Kirchhoff  has  done  carries  us  far  beyond  all  that  had 
before  been  accomplished.  He  has  introduced  the  order 
of  law  amid  a  vast  assemblage  of  empirical  observations, 
and  has  ennobled  our  previous  knowledge  by  showing  its 
relationship  to  some  of  the  most  sublime  of  natural  phe- 
nomena. 


SOIENOB 16 


XT 


ELEMENTARY      MAGNETISM 
A  LECTURE  TO  SCHOOLMASTERS 

WE  have  no  reason  to  believe  that  the  sheep  or  the 
dog,  or  indeed  any  of  the  lower  animals,  feel 
an  interest  in  the  laws  by  which  natural  phe- 
nomena are  regulated.  A  herd  may  be  terrified  by  a 
thunderstorm;  birds  may  go  to  roost,  and  cattle  return 
to  their  stalls,  during  a  solar  eclipse;  but  neither  birds 
nor  cattle,  as  far  as  we  know,  ever  think  of  inquiring  into 
the  causes  of  these  things.  It  is  otherwise  with  man. 
The  presence  of  natural  objects,  the  occurrence  of  natural 
events,  the  varied  appearances  of  the  universe  in  which 
he  dwells  penetrate  beyond  his  organs  of  sense,  and  ap- 
peal to  an  inner  power  of  which  the  senses  are  the  mere 
instruments  and  excitants.  No  fact  is  to  him  either  orig- 
inal or  final.  He  cannot  limit  himself  to  the  contempla- 
tion of  it  alone,  but  endeavors  to  ascertain  its  position 
in  a  series  to  which  uniform  experience  assures  him  it 
must  belong.  He  regards  all  that  he  witnesses  in  the 
present  as  the  efilux  and  sequence  of  something  that  has 
gone  before,  and  as  the  source  of  a  system  of  events  which 
is  to  follow.  The  notion  of  spontaneity,  by  which  in  his 
ruder  state  he  accounted  for  natural  events,  is  abandoned; 
the  idea  that  nature  is  an  aggregate  of  independent  parts 
also  disappears,  as  the  connection  and  mutual  dependence 
(362) 


ELEMENTARY  MAGNETISM  363 

of  physical  powers  become  more  and  more  manifest:  until 
he  is  finally  led  to  regard  Nature  as  an  organic  whole — as 
a  body  each  of  whose  members  sympathizes  with  the  rest, 
changing,  it  is  true,  from  age  to  age,  but  changing  with- 
out break  of  continuity  in  the  relation  of  cause  and  effect. 

The  system  of  things  which  we  call  Nature  is,  how- 
ever, too  vast  and  various  to  be  studied  first-hand  by  any 
single  mind.  As  knowledge  extends  there  is  always  a 
tendency  to  subdivide  the  field  of  investigation.  Its  vari- 
ous parts  are  taken  up  by  different  minds,  and  thus  re- 
ceive a  greater  amount  of  attention  than  could  possibly 
be  bestowed  on  them  if  each  investigator  aimed  at  the 
mastery  of  the  whole.  The  centrifugal  form  in  which 
knowledge,  as  a  whole,  advances,  spreading  ever  wider 
on  all  sides,  is  due  in  reality  to  the  exertions  of  individ- 
uals, each  of  whom  directs  his  efforts,  more  or  less,  along 
a  single  line.  Accepting,  in  many  respects,  his  culture 
from  his  fellow-men —taking  it  from  spoken  words  or  from 
written  books — ^in  some  one  direction,  the  student  of  Nat- 
ure ought  actually  to  touch  his  work.  He  may  otherwise 
be  a  distributor  of  knowledge,  but  not  a  creator,  and  he 
fails  to  attain  that  vitality  of  thought  and  correctness  of 
judgment  which  direct  and  habitual  contact  with  natural 
truth  can  alone  impart. 

One  large  department  of  the  system  of  Nature  which 
forms  the  chief  subject  of  my  own  studies,  and  to  which 
it  is  my  duty  to  call  your  attention  this  evening,  is  that 
of  physics,  or  natural  philosophy.  This  term  is  large 
enough  to  cover  the  study  of  Nature  generally,  but  it 
is  usually  restricted  to  a  department  which,  perhaps,  lies 
closer  to  our  perceptions  than  any  other.  It  deals  with 
the  phenomena  and  laws  of  light  and  heat — with  the  phe- 


864  FRAGMENTS    OF  SCIENCE 

nomena  and  laws  of  magnetism  and  electricity — with  those 
of  sound — with  the  pressures  and  motions  of  liquids  and 
gases,  whether  at  rest  or  in  a  state  of  translation  or  of 
undulation.  The  science  of  mechanics  is  a  portion  of  nat- 
ural philosophy,  though  at  present  so  large  as  to  need  the 
exclusive  attention  of  him  who  would  cultivate  it  pro- 
foundly. Astronomy  is  the  application  of  physics  to  the 
motions  of  the  heavenly  bodies,  the  vastness  of  the  field 
causing  it,  however,  to  be  regarded  as  a  department  in 
itself.  In  chemistry  physical  agents  play  important  parts. 
By  heat  and  light  we  cause  atoms  and  molecules  to  unite 
or  to  fall  asunder.  Electricity  exerts  a  similar  power. 
Through  their  ability  to  separate  nutritive  compounds 
into  their  constituents,  the  solar  beams  build  up  the 
whole  vegetable  world,  and  by  it  the  animal  world.  The 
touch  of  the  self-same  beams  causes  hydrogen  and  chlo- 
rine to  unite  with  sudden  explosion,  and  to  form  by  their 
combination  a  powerful  acid.  Thus  physics  and  chem- 
istry intermingle.  Physical  agents  are,  however,  employed 
by  the  chemist  as  a  means  to  an  end;  while  in  physios 
proper  the  laws  and  phenomena  of  the  agents  themselves, 
both  qualitative  and  quantitative,  are  the  primary  objects 
of  attention. 

My  duty  here  to-night  is  to  spend  an  hour  in  telling 
how  this  subject  is  to  be  studied,  and  how  a  knowledge 
of  it  is  to  be  imparted  to  others.  From  the  domain  of 
physics,  which  would  be  unmanageable  as  a  whole,  I  se- 
lect as  a  sample  the  subject  of  magnetism.  I  might  read- 
ily entertain  you  on  the  present  occasion  with  an  account 
of  what  natural  philosophy  has  accomplished.  I  might 
point  to  those  applications  of  science  of  which  we  hear 
so  much  in  the  newspapers,  and  which  are  so  often  mis- 


ELEMENTARY  MAGNETISM  866 

taken  for  science  itself.  I  might,  of  course,  ring  changes 
on  the  steam-engine  and  the  telegraph,  the  electrotype  and 
the  photograph,  the  medical  applications  of  physics,  and 
the  various  other  inlets  by  which  scientific  thought  filters 
into  practical  life.  That  would  be  easy  compared  with 
the  task  of  informing  you  how  you  are  to  make  the  study 
of  physics  the  instrument  of  your  pupil's  culture;  how 
you  are  to  possess  its  facts  and  make  them  living  seeds 
which  shall  take  root  and  grow  in  the  mind,  and  not  lie 
like  dead  lumber  in  the  storehouse  of  memory.  This  is  a 
task  much  heavier  than  the  mere  recounting  of  scientific 
achievements;  and  it  is  one  which,  feeling  my  own  want 
of  time  to  execute  it  aright,  I  might  well  hesitate  to 
accept. 

But  let  me  sink  excuses,  and  attack  the  work  before 
me.  First  and  foremost,  then,  I  would  advise  you  to  get 
a  knowledge  of  facts  from  actual  observation.  Facts 
looked  at  directly  are  vital;  when  they  pass  into  words 
half  the  sap  is  taken  out  of  them.  You  wish,  for  exam- 
ple, to  get  a  knowledge  of  magnetism;  well,  provide  your- 
self with  a  good  book  on  the  subject,  if  you  can,  but  do 
not  be  content  with  what  the  book  tells  you;  do  not  be 
satisfied  with  its  descriptive  woodcuts;  see  the  operations 
of  the  force  yourself.  Half  of  our  book  writers  describe 
experiments  which  they  never  made,  and  their  descrip- 
tions often  lack  both  force  and  truth;  but,  no  matter  how 
clever  or  conscientious  they  may  be,  their  written  words 
cannot  supply  the  place  of  actual  observation.  Every  fact 
has  numerous  radiations,  which  are  shorn  off  by  the  man 
who  describes  it.  Go,  then,  to  a  philosophical  instrument 
maker,  and  give  a  shilling  or  half  a  crown  for  a  straight 
bar-magnet,  or,  if  you  can  afford  it,  purchase  a  pair  of 


FRAGMENTS   OF  SCIENCE 

them;  or  get  a  smith  to  cut  a  length  of  ten  inches  from 
a  bar  of  steel  an  inch  wide  and  half  an  inch  thick;  file 
its  ends  smoothly,  harden  it,  and  get  somebody  like  my- 
self to  magnetize  it.  Procure  some  darning  needles,  and 
also  a  little  unspun  silk,  which  will  give  you  a  suspend- 
ing fibre  void  of  torsion.  Make  a  little  loop  of  paper,  or 
of  wire,  and  attach  your  fibre  to  it.  Do  it  neatly.  In  the 
loop  place  a  darning-needle,  and  bring  the  two  ends  or 
poles,  as  they  are  called,  of  your  bar-magnet  successively 
up  to  the  ends  of  the  needle.  Both  the  poles,  you  find, 
attract  both  ends  of  the  needle.  Eeplace  the  needle  by  a 
bit  of  annealed  iron  wire;  the  same  effects  ensue.  Sus- 
pend successively  little  rods  of  lead,  copper,  silver,  brass, 
wood,  glass,  ivory,  or  whalebone;  the  magnet  produces  no 
sensible  effect  upon  any  of  the  substances.  You  thence 
infer  a  special  property  in  the  case  of  steel  and  iron. 
Multiply  your  experiments,  however,  and  you  will  find 
that  some  other  substances,  besides  iron  and  steel,  are 
acted  apon  by  your  magnet.  A  rod  of  the  metal  nickel, 
or  of  the  metal  cobalt,  from  which  the  blue  color  used 
by  painters  is  derived,  exhibits  powers  similar  to  those 
observed  with  the  iron  and  steel. 

In  studying  the  character  of  the  force  you  may,  how- 
ever, confine  yourself  to  iron  and  steel,  which  are  always 
at  hand.  Make  your  experiments  with  the  darning-needle 
over  and  over  again;  operate  on  both  ends  of  the  needle; 
try  both  ends  of  the  magnet.  Do  not  think  the  work 
dull;  you  are  conversing  with  Nature,  and  must  acquire 
over  her  language  a  certain  grace  and  mastery,  which 
practice  can  alone  impart.  Let  every  movement  be  made 
with  care,  and  avoid  slovenliness  from  the  outset.  Ex- 
periment,  as  I  have  said,   is  the  language  by  which  we 


ELEMENTARY  MAGNETISM  367 

address  Nature,  and  through  which  she  sends  her  replies; 
in  the  use  of  this  language  a  lack  of  straightforwardness 
is  as  possible,  and  as  prejudicial,  as  in  the  spoken  lan- 
guage of  the  tongue.  If,  therefore,  you  wish  to  become 
acquainted  with  the  truth  of  Nature,  you  must  from  the 
first  resolve  to  deal  with  her  sincerely. 

Now  remove  your  needle  from  its  loop,  and  draw  it, 
from  eye  to  point,  along  one  of  the  ends  of  the  magnet; 
resuspend  it,  and  repeat  your  former  experiment.  You 
now  find  that  each  extremity  of  the  magnet  attracts  one 
end  of  the  needle  and  repels  the  other.  The  simple  at- 
traction observed  in  the  first  instance  is  now  replaced 
by  a  dual  force.  Repeat  the  experiment  till  you  have 
thoroughly  observed  the  ends  which  attract  and  those 
which  repel  each  other. 

Withdraw  the  magnet  entirely  from  the  vicinity  of 
your  needle,  and  leave  the  latter  freely  suspended  by  its 
fibre.  Shelter  it  as  well  as  you  can  from  currents  of  air, 
and  if  you  have  iron  buttons  on  your  coat,  or  a  steel 
penknife  in  your  pocket,  beware  of  their  action.  If  you 
work  at  night,  beware  of  iron  candlesticks,  or  of  brass 
ones  with  iron  rods  inside.  Freed  from  such  disturb- 
ances, the  needle  takes  up  a  certain  determinate  position. 
It  sets  its  length  nearly  north  and  south.  Draw  it  aside 
and  let  it  go.  After  several  oscillations  it  will  again 
come  to  the  same  position.  If  you  have  obtained  your 
magnet  from  a  philosophical  instrument  maker,  you  will 
see  a  mark  on  one  of  its  ends.  Supposing,  then,  that 
you  drew  your  needle  along  the  end  thus  marked,  and 
that  the  point  of  your  needle  was  the  last  to  quit  the 
magnet,  you  will  find  that  the  point  turns  to  the  south, 
the  eye  of  the   needle  turning  toward  the  north.      Make 


368  FRAGMENTS   OF  SCIENCE 

sure  of  this,  and  do  not  take  the  statement  on  my 
authority. 

Now,  take  a  second  darning-needle  like  the  first,  and 
magnetize  it  in  precisely  the  same  manner:  freely  sus- 
pended it  also  will  turn  its  eye  to  the  north  and  its  point 
to  the  south.  Your  next  step  is  to  examine  the  action  of 
the  two  needles,  which  you  have  thus  magnetized,  upon 
each  other. 

Take  one  of  them  in  your  hand,  and  leave  the  other 
suspended;  bring  the  eye-end  of  the  former  near  the  eye- 
end  of  the  latter;  the  suspended  needle  retreats;  it  is  re- 
pelled. Make  the  same  experiment  with  the  two  points; 
you  obtain  the  same  result,  the  suspended  needle  is  re- 
pelled. Now  cause  the  dissimilar  ends  to  act  on  each 
other — ^you  have  attraction — point  attracts  eye,  and  eye 
attracts  point.  Prove  the  reciprocity  of  this  action  by 
removing  the  suspended  needle,  and  putting  the  other  in 
its  place.  You  obtain  the  same  result.  The  attraction, 
then,  is  mutual,  and  the  repulsion  is  mutual.  You  have 
thus  demonstrated  in  the  clearest  manner  the  fundamental 
law  of  magnetism,  that  like  poles  repel,  and  that  unlike 
poles  attract,  each  other.  You  may  say  that  this  is  all 
easily  understood  without  doing;  but  do  it,  and  your 
knowledge  will  not  be  confined  to  what  I  have  uttered 
here. 

I  have  said  that  one  end  of  your  bar-magnet  has  a 
mark  upon  it;  lay  several  silk  fibres  together,  so  as  to  get 
sufficient  strength,  or  employ  a  thin  silk  ribbon,  and  form 
a  loop  large  enough  to  hold  your  magnet.  Suspend  it;  it 
turns  ils  marked  end  toward  the  north.  This  marked  end 
is  that  which  in  England  is  called  the  north  pole.  If  a 
common  smith  has  made  your  magnet,  it  will  be  conven- 


ELEMENTARY  MAGNETISM  869 

lent  to  determine  its  north  pole  yourself,  and  to  mark 
it  with  a  file.  Vary  your  experiments  by  causing  your 
magnetized  darning-needle  to  attract  and  repel  your  large 
magnet;  it  is  quite  competent  to  do  so.  In  magnetizing 
the  needle,  I  have  supposed  the  point  to  be  the  last  to 
quit  the  marked  end  of  the  magnet;  the  point  of  the 
needle  is  a  south  pole.  The  end  which  last  quits  the 
magnet  is  always  opposed  in  polarity  to  the  end  of 
the  magnet  with  which  it  has  been  last  in  contact. 

You  may  perhaps  learn  all  this  in  a  single  hour;  but 
spend  several  at  it,  if  necessary;  and  remember,  under- 
standing it  is  not  sufficient:  you  must  obtain  a  manual 
aptitude  in  addressing  Nature.  If  you  speak  to  your 
fellow-man  you  are  not  entitled  to  use  jargon.  Bad  ex- 
periments are  jargon  addressed  to  Nature,  and  just  as 
much  to  be  deprecated.  Manual  dexterity  in  illustrating 
the  interaction  of  magnetic  poles  is  of  the  utmost  impor- 
tance at  this  stage  of  your  progress;  and  you  must  not 
neglect  attaining  this  power  over  your  implements.  As 
you  proceed,  moreover,  you  will  be  tempted  to  do  more 
than  I  can  possibly  suggest.  Thoughts  will  occur  to  you 
which  you  will  endeavor  to  follow  out:  questions  will  arise 
which  you  will  try  to  answer.  The  same  experiment  may 
be  twenty  different  things  to  twenty  people.  Having  wit- 
nessed the  action  of  pole  on  pole,  through  the  air,  you 
will  perhaps  try  whether  the  magnetic  power  is  not  to  be 
screened  off.  You  use  plates  of  glass,  wood,  slate,  paste- 
board, or  gutta-percha,  but  find  them  all  pervious  to  this 
wondrous  force.  One  magnetic  pole  acts  upon  another 
through  these  bodies  as  if  they  were  not  present.  Should 
you  ever  become  a  patentee  for  the  regulation  of  ships' 
compasses,    you   will    not    fall,    as    some    projectors    have 


870  FRAGMENTS    OF  SCIENCE 

done,  into  the  error  of  screening  off  the  magnetism  of  the 
ship  by  the  interposition  of  such  substances. 

If  you  wish  to  teach  a  class  you  must  contrive  that  the 
effects  which  you  have  thus  far  witnessed  for  yourself 
shall  be  witnessed  by  twenty  or  thirty  pupils.  And  here 
your  private  ingenuity  must  come  into  play.  You  will 
attach  bits  of  paper  to  your  needles,  so  as  to  render  their 
movements  visible  at  a  distance,  denoting  the  north  and 
south  poles  by  different  colors,  say  green  and  red.  You 
may  also  improve  upon  your  darning-needle.  Take  a 
strip  of  sheet  steel,  heat  it  to  vivid  redness  and  plunge 
it  into  cold  water.  It  is  thereby  hardened ;  rendered,  in 
fact,  almost  as  brittle  as  glass.  Six  inches  of  this,  mag- 
netized in  the  manner  of  the  darning-needle,  will  be  bet- 
ter able  to  carry  your  paper  indexes.  Having  secured 
such  a  strip,  you  proceed  thus: 

Magnetize  a  small  sewing-needle  and  determine  its 
poles ;  or,  break  half  an  inch,  or  an  inch,  off  your  mag- 
netized darning-needle  and  suspend  it  by  a  fine  silk  fibre. 
The  sewing-needle,  or  the  fragment  of  the  darning  needle, 
is  now  to  be  used  as  a  test-needle,  to  examine  the  distribu- 
tion of  the  magnetism  in  your  strip  of  steel.  Hold  the 
strip  upright  in  your  left  hand,  and  cause  the  test-needle 
to  approach  the  lower  end  of  your  strip;  one  end  of  the 
test-needle  is  attracted,  the  other  is  repelled.  Eaise  your 
needle  along  the  strip;  its  oscillations,  which  at  first  were 
quick,  become  slower;  opposite  the  middle  of  the  strip 
they  cease  entirely;  neither  end  of  the  needle  is  attracted; 
above  the  middle  the  test-needle  turns  suddenly  round,  its 
other  end  being  now  attracted.  Go  through  the  experi- 
ment thoroughly:  you  thus  learn  that  the  entire  lower 
half  of  the  strip  attracts  one  end  of  the  needle,  while  the 


ELEMENTARY  MAGNETISM  871 

entire  upper  half  attracts  the  opposite  end.  Supposing 
the  north  end  of  your  little  needle  to  be  that  attracted 
below,  you  infer  that  the  entire  lower  half  of  your  mag- 
netized strip  exhibits  south  magnetism,  while  the  entire 
upper  half  exhibits  north  magnetism.  So  far,  then,  you 
have  determined  the  distribution  of  magnetism  in  your 
strip  of  steel. 

You  look  at  this  fact,  you  think  of  it;  in  its  sugges- 
tiveness  the  value  of  an  experiment  chiefly  consists.  The- 
thought  naturally  arises:  "What  will  occur  if  I  break  my 
strip  of  steel  across  in  the  middle?  Shall  I  obtain  two 
magnets  each  possessing  a  single  pole?"  Try  the  experi- 
ment; break  your  strip  of  steel,  and  test  each  half  as  you 
tested  the  whole.  The  mere  presentation  of  its  two  ends 
in  succession  to  your  test-needle  suffices  to  show  that  you 
have  not  a  magnet  with  a  single  pole — that  each  half  pos- 
sesses two  poles  with  a  neutral  point  between  them.  And 
if  you  again  break  the  half  into  two  other  halves,  you 
will  find  that  each  quarter  of  the  original  strip  exhibits 
precisely  the  same  magnetic  distribution  as  the  whole 
strip.  You  may  continue  the  breaking  process:  no  mat- 
ter how  small  your  fragment  may  be,  it  still  possesses 
two  opposite  poles  and  a  neutral  point  between  them. 
Well,  your  hand  ceases  to  break  where  breaking  becomes 
a  mechanical  impossibility;  but  does  the  mind  stop  there? 
No:  you  follow  the  breaking  process  in  idea  when  you 
can  no  longer  realize  it  in  fact;  your  thoughts  wander 
amid  the  very  atoms  of  your  steel,  and  you  conclude  that 
each  atom  is  a  magnet,  and  that  the  force  exerted  by  the 
strip  of  steel  is  the  mere  summation,  or  resultant,  of  the 
forces  of  its  ultimate  particles. 

Here,  then,  is  an   exhibition  of   power  which  we  can 


872  FRAGMENTS   OF  SCIENCE 

call  forth  at  pleasure  or  cause  to  disappear.  We  magne- 
tize our  strip  of  steel  by  drawing  it  along  the  pole  of  a 
magnet;  we  can  demagnetize  it,  or  reverse  its  magnetism, 
by  properly  drawing  it  along  the  same  pole  in  the  oppo- 
site direction.  What,  then,  is  the  real  nature  of  this 
wondrous  change?  What  is  it  that  takes  place  among  the 
atoms  of  the  steel  when  the  substance  is  magnetized?  The 
question  leads  us  beyond  the  region  of  sense,  and  into 
that  of  imagination.  This  faculty,  indeed,  is  the  divining- 
rod  of  the  man  of  science.  Not,  however,  an  imagination 
which  catches  its  creations  from  the  air,  but  one  informed 
and  inspired  by  facts;  capable  of  seizing  firmly  on  a 
physical  image  as  a  principle,  of  discerning  its  conse- 
quences, and  of  devising  means  whereby  these  forecasts 
of  thought  may  be  brought  to  an  experimental  test.  If 
such,  a  principle  be  adequate  to  account  for  all  the  phe- 
nomena— ^if,  from  an  assumed  cause,  the  observed  acts 
necessarily  follow,  we  call  the  assumption  a  theory,  and, 
once  possessing  it,  we  can  not  only  revive  at  pleasure 
facts  already  known,  but  we  can  predict  others  which  we 
have  never  seen.  Thus,  then,  in  the  prosecution  of  phys- 
ical science,  our  powers  of  observation,  memory,  imagina- 
tion, and  inference,  are  all  drawn  upon.  We  observe 
facts  and  store  them  up;  the  constructive  imagination 
broods  upon  these  memories,  tries  to  discern  their  inter- 
dependence and  weave  them  to  an  organic  whole.  The 
theoretic  principle  flashes  or  slowly  dawns  upon  the  mind; 
and  then  the  deductive  faculty  interposes  to  carry  out  the 
principle  to  its  logical  consequences.  A  perfect  theory 
gives  dominion  over  natural  facts;  and  even  an  assump- 
tion which  can  only  partially  stand  the  test  of  a  compar- 
ison with  facts  may  be  of  eminent  use  in  enabling  us  to 


ELEMENTARY  MAGNETISM  873 

connect  and  classify  groups  of  phenomena.  The  theory 
of  magnetic  fluids  is  of  this  latter  character,  and  with  it 
we  must  now  make  ourselves  familiar. 

With  the  view  of  stamping  the  thing  more  firmly  on 
your  minds,  I  will  make  use  of  a  strong  and  vivid  image. 
In  optics,  red  and  green  are  called  complementary  colors; 
their  mixture  produces  white.  Now  I  ask  you  to  imagine 
each  of  these  colors  to  possess  a  self -repulsive  power;  that 
red  repels  red,  that  green  repels  green;  but  that  red  at- 
tracts green  and  green  attracts  red,  the  attraction  of  the 
dissimilar  colors  being  equal  to  the  repulsion  of  the  sim- 
ilar ones.  Imagine  the  two  colors  mixed  so  as  to  produce 
white,  and  suppose  two  strips  of  wood  painted  with  this 
white;  what  will  be  their  action  upon  each  other?  Sus- 
pend one  of  them  freely  as  we  suspended  our  darning- 
needle,  and  bring  the  other  near  it;  what  will  occur? 
The  red  component  of  the  strip  you  hold  in  your  hand 
will  repel  the  red  component  of  your  suspended  strip; 
but  then  it  will  attract  the  green,  and,  the  forces  being 
equal,  they  neutralize  each  other.  In  fact,  the  least  re- 
flection shows  you  that  the  strips  will  be  as  indifferent 
to  each  other  as  two  unmagnetized  darning-needles  would 
be  under  the  same  circumstances. 

But  suppose,  instead  of  mixing  the  colors,  we  painted 
one  half  of  each  strip  from  centre  to  end  red,  and  the 
other  half  green,  it  is  perfectly  manifest  that  the  two 
strips  would  now  behave  toward  each  other  exactly  as 
our  two  magnetized  darning-needles — ^the  red  end  would 
repel  the  red  and  attract  the  green,  the  green  would  repel 
the  green  and  attract  the  red;  so  that,  assuming  two  col- 
ors thus  related  to  each  other,  we  could  by  their  mixture 
produce   the  neutrality  of   an    unmagnetized    body,   while 


374  FRAGMENTS   OF  SCIENCE 

by  their  separation  we  could  produce  the  duality  of  action 
of  magnetized  bodies. 

But  you  have  already  anticipated  a  defect  in  my  con- 
ception; for  if  we  break  one  of  our  strips  of  wood  in  the 
middle  we  have  one  half  entirely  red,  and  the  other  en- 
tirely green,  and  with  these  it  would  be  impossible  to 
imitate  the  action  of  our  broken  magnet.  How,  then, 
must  we  modify  our  conception?  We  must  evidently  sup- 
pose each  molecule  of  the  wood  painted  green  on  one  face 
and  red  on  the  opposite  one.  The  resultant  action  of  all 
the  atoms  would  then  exactly  resemble  the  action  of  a 
magnet.  Here  also,  if  the  two  opposite  colors  of  each 
atom  could  be  caused  to  mix  so  as  to  produce  white,  we 
should  have,  as  before,  perfect  neutrality. 

For  these  two  self -repellent  and  mutually  attractive 
colors  substitute  in  your  minds  two  invisible  self-repel- 
lent and  mutually  attractive  fluids,  which  in  ordinary 
steel  are  mixed  to  form  a  neutral  compound,  but  which 
the  act  of  magnetization  separates  from  each  other,  plac^ 
ing  the  opposite  fluids  on  the  opposite  face  of  each  mole- 
cule. You  have  then  a  perfectly  distinct  conception  of 
the  celebrated  theory  of  magnetic  fluids.  The  strength 
of  the  magnetism  excited  is  supposed  to  be  proportional 
to  the  quantity  of  neutral  fluid  decomposed.  Accord- 
ing to  this  theory  nothing  is  actually  transferred  from  the 
exciting  magnet  to  the  excited  steel.  The  act  of  mag- 
netization consists  in  the  forcible  separation  of  two  fluids 
which  existed  in  the  steel  before  it  was  magnetized,  but 
which  then  neutralized  each  other  by  their  coalescence. 
And  if  you  test  your  magnet,  after  it  has  excited  a  hun- 
dred pieces  of  steel,  you  will  find  that  it  has  lost  no  force 
— ^no  more,  indeed,  than  I  should  lose,  had  my  words  such 


ELEMENTARY  MAGNETISM  376 

a  magnetic  influence  on  your  minds  as  to  excite  in  them  a 
strong  resolve  to  study  natural  philosophy.  I  should  rather 
be  the  gainer  by  my  own  utterance,  and  by  the  reaction  of 
your  fervor.  The  magnet  also  is  the  gainer  by  the  reac- 
tion of  the  body  which  it  magnetizes. 

Look  now  to  your  excited  piece  of  steel;  figure  each 
molecule  with  its  opposed  fluids  spread  over  its  opposite 
faces.  How  can  this  state  of  things  be  permanent?  The 
fluids,  by  hypothesis,  attract  each  other;  what,  then,  keeps 
them  apart?  Why  do  they  not  instantly  rush  together 
across  the  equator  of  the  atom,  and  thus  neutralize  each 
other?  To  meet  this  question,  philosophers  have  been 
obliged  to  infer  the  existence  of  a  special  force,  which 
holds  the  fluids  asunder.  They  call  it  coercive  force;  and 
it  is  found  that  those  kinds  of  steel  which  offer  most  re- 
sistance to  being  magnetized — which  require  the  greatest 
amount  of  * 'coercion"  to  tear  their  fluids  asunder — are  the 
very  ones  which  offer  the  greatest  resistance  to  the  re- 
union of  the  fluids  after  they  have  been  once  separated. 
Such  kinds  of  steel  are  most  suited  to  the  formation  of 
permanent  magnets.  It  is  manifest,  indeed,  that  without 
coercive  force  a  permanent  magnet  would  not  be  at  all 
possible. 

Probably  long  before  this  you  will  have  dipped  the 
end  of  your  magnet  among  iron  filings,  and  observed  how 
they  cling  to  it;  or  into  a  nail-box,  and  found  how  it 
drags  the  nails  after  it.  I  know  very  well  that  if  you 
are  not  the  slaves  of  routine  you  will  have  by  this  time 
done  many  things  that  I  have  not  told  you  to  do,  and 
thus  multiplied  your  experience  beyond  what  I  have  in- 
dicated. You  are  almost  sure  to  have  caused  a  bit  of 
iron  to  hang  from  the  end  of  your  magnet,  and  you  have 


876  FRAGMENTS   OF  SCIENCE 

probably  succeeded  in  causing  a  second  bit  to  attach  itself 
to  the  first,  a  third  to  the  second;  until  finally  the  force 
has  become  too  feeble  to  bear  the  weight  of  more.  If 
you  have  operated  with  nails,  you  may  have  observed 
that  the  points  and  edges  hold  together  with  the  greatest 
tenacity;  and  that  a  bit  of  iron  clings  more  firmly  to  the 
corner  of  your  magnet  than  to  one  of  its  fiat  surfaces.  In 
short,  you  will  in  all  likelihood  have  enriched  your  expe- 
rience in  many  ways  without  any  special  direction  from  me. 

Well,  the  magnet  attracts  the  nail,  and  the  nail  attracts 
a  second  one.  This  proves  that  the  nail  in  contact  with 
the  magnet  has  had  the  magnetic  quality  developed  in  it 
by  that  contact.  If  it  be  withdrawn  from  the  magnet  its 
power  to  attract  its  fellow  nail  ceases.  Contact,  however, 
is  not  necessary.  A  sheet  of  glass  or  paper,  or  a  space 
of  air,  may  exist  between  the  magnet  and  the  nail;  the 
latter  is  still  magnetized,  though  not  so  forcibly  as  when 
in  actual  contact.  The  nail  thus  presented  to  the  magnet 
is  itself  a  temporary  magnet.  That  end  which  is  turned 
toward  the  magnetic  pole  has  the  opposite  magnetism  of 
the  pole  which  excites  it;  the  end  most  remote  from  the 
pole  has  the  same  magnetism  as  the  pole  itself,  and  be- 
tween the  two  poles  the  nail,  like  the  magnet,  possesses 
a  magnetic  equator. 

Conversant  as  you  now  are  with  the  theory  of  magnetic 
fluids,  you  have  already,  I  doubt  not,  anticipated  me  in 
imagining  the  exact  condition  of  an  iron  nail  under  the 
influence  of  the  magnet.  You  picture  the  iron  as  pos- 
sessing the  neutral  fluid  in  abundance;  you  picture  the 
magnetic  pole,  when  brought  near,  decomposing  the  fluid; 
repelling  the  fluid  of  a  like  kind  with  itself,  and  attract- 
ing the  unlike  fluid;    thus   exciting  in  the  parts  of  the 


ELEMENTARY  MAGNETISM  377 

iron  nearest  to  itself  the  opposite  polarity.  But  the  iron 
is  incapable  of  becoming  a  permanent  magnet.  It  only 
shows  its  virtue  as  long  as  the  magnet  acts  upon  it. 
What,  then,  does  the  iron  lack  which  the  steel  possesses? 
It  lacks  coercive  force.  Its  fluids  are  separated  with  ease; 
but,  once  the  separating  cause  is  removed,  they  flow  to- 
gether again,  and  neutrality  is  restored.  Imagination  must 
be  quite  nimble  in  picturing  these  changes — able  to  see 
the  fluids  dividing  and  reuniting,  according  as  the  magnet 
is  brought  near  or  withdrawn.  Fixing  a  definite  pole  in 
your  mind,  you  must  picture  the  precise  arrangement  of 
the  two  fluids  with  reference  to  this  pole,  and  be  able 
to  arouse  similar  pictures  in  the  minds  of  your  pupils. 
You  will  cause  them  to  place  magnets  and  iron  in  vari- 
ous positions,  and  describe  the  exact  magnetic  state  of 
the  iron  in  each  particular  case.  The  mere  facts  of  mag- 
netism will  have  their  interest  immensely  augmented  by 
an  acquaintance  with  the  principles  whereon  the  facts  de- 
pend. Still,  while  you  use  this  theory  of  magnetic  fluids 
to  track  out  the  phenomena  and  link  them  together,  you 
will  not  forget  to  tell  your  pupils  that  it  is  to  be  regarded 
as  a  symbol  merely — a  symbol,  moreover,  which  is  incom- 
petent to  cover  all  the  facts,'  but  which  does  good  prac- 
tical service  while  we  are  waiting  for  the  actual  truth. 

The  state  of  excitement  into  which  iron  is  thrown  by 
the  influence  of  a  magnet  is  sometimes  called  *' magneti- 
zation by  influence.'*     More  commonly,  however,  the  mag- 


*  This  theory  breaks  down  when  applied  to  diamagnetic  bodies  which  are 
repelled  by  magnets.  Like  soft  iron,  such  bodies  are  thrown  into  a  state  of 
temporary  excitement,  in  virtue  of  which  they  are  repelled;  but  any  attempt 
to  explain  such  a  repulsion  by  the  decomposition  of  a  fluid  will  demonstrate 
itft  own  futility. 


878  FRAGMENTS   OF  SCIENCE 

netism  is  said  to  be  "induced"  in  the  iron,  and  hence 
this  mode  of  magnetizing  is  called  "magnetic  induction." 
Now,  there  is  nothing  theoretically  perfect  in  Nature: 
there  is  no  iron  so  soft  as  not  to  possess  a  certain 
amount  of  coercive  force,  and  no  steel  so  hard  as  not  to 
be  capable,  in  some  degree,  of  magnetic  induction.  The 
quality  of  steel  is  in  some  measure  possessed  by  iron, 
and  the  quality  of  iron  is  shared  in  some  degree  by  steel. 
It  is  in  virtue  of  this  latter  fact  that  the  unmagnetized 
darning-needle  was  attracted  in  your  first  experiment; 
and  from  this  you  may  at  once  deduce  the  consequence 
that,  after  the  steel  has  been  magnetized,  the  repulsive 
action  of  a  magnet  must  be  always  less  than  its  attractive 
action.  For  the  repulsion  is  opposed  by  the  inductive 
action  of  the  magnet  on  the  steel,  while  the  attraction  is 
assisted  by  the  same  inductive  action.  Make  this  clear 
to  your  minds,  and  verify  it  by  your  experiments.  In 
some  cases  you  can  actually  make  the  attraction  due  to 
the  temporary  magnetism  overbalance  the  repulsion  due 
to  the  permanent  magnetism,  and  thus  cause  two  poles  of 
the  same  kind  apparently  to  attract  each  other.  When, 
however,  good  hard  magnets  act  on  each  other  from  a 
sufficient  distance,  the  inductive  action  practically  van- 
ishes, and  the  repulsion  of  like  poles  is  sensibly  equal 
to  the  attraction  of  unlike  ones. 

I  dwell  thus  long  on  elementary  principles,  because 
they  are  of  the  first  importance,  and  it  is  the  tempta- 
tion of  this  age  of  unhealthy  cramming  to  neglect  them. 
Now  follow  me  a  little  further.  In  examining  the  distri- 
bution of  magnetism  in  your  strip  of  steel  you  raised  the 
needle  slowly  from  bottom  to  top,  and  found  what  we 
called  a  neutral  point  at  the  centre.     Now,  does  the  mag- 


ELEMENTARY  MAGNETISM  379 

net  really  exert  no  influence  on  the  pole  presented   (o  its 
centre?     Let  us  see. 

Let  s  N,  Fig.  11,  be  our  magnet,  and  let  n  represent  a 
particle  of  north,  magnetism  placed  exactly  opposite  the 
middle  of  the  magnet.  Of  course  this  is  an  imaginary 
case,  as  you  can  never  in  reality  thus  detach  your  north 
magnetism  from  its  neighbor.  But,  supposing  us  to  hare 
done  so,  what  would  be  the  action  of  the  two  poles  of 
the  magnet  on  n'l  Your  reply  will,  of  course,  be  that 
the  pole  s  attracts  n  while  the  pole  N  repels  it.  Let  the 
magnitude  and  direction  of  the  attraction  be  expressed  by 


Fig.  11. 

the  line  n  m,  and  the  magnitude  and  direction  of  the 
repulsion  by  the  line  n  o.  Now,  the  particle  n  being 
equally  distant  from  8  and  N,  the  line  n  o,  expressing 
the  repulsion,  will  be  equal  to  m  n,  which  expresses  the 
attraction.  Acted  upon  by  two  such  forces,  the  particle  n 
must  evidently  move  in  the  direction  n  p^  exactly  mid- 
way between  m  n  and  n  o.  Hence  you  see  that,  although 
there  is  no  tendency  of  the  particle  n  to  move  toward  the 
magnetic  equator,  there  is  a  tendency  on  its  part  to  move 
parallel  to  the  magnet.  If,  instead  of  a  particle  of  north 
magnetism,  we  placed  a  particle  of  south  magnetism  op- 
posite to  the  magnetic  equator,  it  would  evidently  be 
urged  along  the  line  n  q\    and  if,  instead  of  two  separate 


880  FRAGMENTS   OF  SCIENCE 

particles  of  magnetism,  we  place  a  little  magnetic  needle, 
containing  both  north  and  south  magnetism,  opposite  the 
magnetic  equator,  its  south  pole  being  urged  along  n  q, 
and  its  north  along  n  p,  the  little  needle  will  be  com- 
pelled to  set  itself  parallel  to  the  magnet  s  N.  Make 
the  experiment,  and  satisfy  yourselves  that  this  is  a  true 
deduction. 

Substitute  for  your  magnetic  needle  a  bit  of  iron  wire, 
devoid  of  permanent  magnetism,  and  it  will  set  itself  ex- 
actly as  the  needle  does.  Acted  upon  by  the  magnet,  the 
wire,  as  you  know,  becomes  a  magnet  and  behaves  as 
such ;  it  will  turn  its  north  pole  toward  p,  and  south  pole 
toward  q,  just  like  the  needle. 

But  supposing  you  shift  the  position  of  your  particle 
of  north  magnetism,  and  bring  it  nearer  to  one  end  of 
your  magnet  than  to  the  other;  the  forces  acting  on  the 
particle  are  no  longer  equal;  the  nearest  pole  of  the  mag- 
net will  act  more  powerfully  on  the  particle  than  the  more 
distant  one.  Let  s  N,  Fig.  12,  be  the  magnet,  and  n  the 
particle  of  north  magnetism,  in  its  new  position.  It  is 
repelled  by  N,  and  attracted  by  s.  Let  the  repulsion  be 
represented  in  magnitude  and  direction  by  the  line  n  o, 
and  the  attraction  by  the  shorter  line  n  m.  The  resultant 
of  these  two  forces  will  be  found  by  completing  the  par- 
allelogram m  n  o  p^  and  drawing  its  diagonal  n  p.  Along 
n  p,  then,  a  particle  of  north  magnetism  would  be  urged 
by  the  simultaneous  action  of  s  and  N.  Substituting  a 
particle  of  south  magnetism  for  n,  the  same  reasoning 
would  lead  to  the  conclusion  that  the  particle  would  be 
urged  along  n  q.  If  we  place  a.i  n  a.  short  magnetic  nee- 
dle, its  north  pole  will  be  urged  along  n  p,  its  south  pole 
along  n  q,  the  only  position  possible  to  the  needle,   thus 


ELEMENTARY   MAGNETISM  381 

acted  on,  being  along  the  line  p  q,  which  is  no  longer 
parallel  to  the  magnet.  Verify  this  deduction  by  actual 
experiment. 

In  this  way  we  might  go  round  the  entire  magnet; 
and,  considering  its  two  poles  as  two  centres  from  which 
the  force  emanates,  we  could,  in  accordance  with  ordinary- 
mechanical  principles,  assign  a  definite  direction  to  the 
magnetic  needle  at  every  particular  place.  And  substi- 
tuting, as  before,  a  bit  of  iron  wire  for  the  magnetic 
needle,  the  positions  of  both  will  be  the  same. 

Now,  I  think,  without  further  preface,  you  will  be  able 


Fia.  12. 

to  comprehend  for  yourselves,  and  explain  to  others,  one 
of  the  most  interesting  effects  in  the  whole  domain  of 
magnetism.  Iron  filings  you  know  are  particles  of  iron, 
irregular  in  shape,  being  longer  in  some  directions  than 
in  others.  For  the  present  experiment,  moreover,  instead 
of  the  iron  filings,  very  small  scraps  of  thin  iron  wire 
might  be  employed.  I  place  a  sheet  of  paper  over  the 
magnet;  it  is  all  the  better  if  the  paper  be  stretched  on  a 
wooden  frame,  as  this  enables  us  to  keep  it  quite  level. 
I  scatter  the  filings,  or  the  scraps  of  wire,  from  a  sieve 
upon  the  paper,  and  tap  the  latter  gently,  so  as  to  liberate 
the  particles  for  a  moment  from  its  friction.  The  magnet 
acts  on  the  filings  through  the  paper,  and  see  how  it  ar- 


382  FRAGMENTS   OF  SCIENCE 

ranges  them!  They  embrace  the  magnet  in  a  series  of 
beautiful  curves,  which  are  technically  called  **  magnetic 
curves,"  or  '* lines  of  magnetic  force."  Does  the  meaning 
of  these  lines  yet  flash  upon  you?  Set  your  magnetic 
needle,  or  your  suspended  bit  of  wire,  at  any  point  of 
one  of  the  curves,  and  you  will  find  the  direction  of  the 
needle,  or  of  the  wire,  to  be  exactly  that  of  the  particle 
of  iron,  or  of  the  magnetic  curve,  at  that  point.  Go 
round  and  round  the  magnet;  the  direction  of  your  nee- 
dle always  coincides  with  the  direction  of  the  curve  od 
which  it  is  placed.  These,  then,  are  the  lines  along 
which  a  particle  of  south  magnetism,  if  you  could  detach 
it,  would  move  to  the  north  pole,  and  a  bit  of  north  mag- 
netism to  the  south  pole.  They  are  the  lines  along  which 
the  decomposition  of  the  neutral  fluid  takes  place.  In  the 
case  of  the  magnetic  needle,  one  of  its  poles  being  urged 
in  one  direction,  and  the  other  pole  in  the  opposite  direc- 
tion, the  needle  must  necessarily  set  itself  as  a  tangent 
to  the  curve. 

I  will  not  seek  to  simplify  this  subject  further.  If 
there  be  anything  obscure  or  confused  or  incomplete 
in  my  statement,  you  ought  now,  by  patient  thought, 
to  be  able  to  clear  away  the  obscurity,  to  reduce  the 
confusion  to  order,  and  to  supply  what  is  needed  to  ren- 
der the  explanation  complete.  Do  not  quit  the  subject 
until  you  thoroughly  understand  it;  and  if  you  are  then 
able  to  look  with  your  mind's  eye  at  the  play  of  forces 
around  a  magnet,  and  see  distinctly  the  operation  of  those 
forces  in  the  production  of  the  magnetic  curves,  the  time 
which  we  have  spent  together  will  not  have  been  spent 
in  vain. 

In  this  thorough  manner  we  must  master  our  materials, 


ELEMENTARY  MAGNETISM 


383 


Ew.  13.— Magnetic  Lines  of  Force  (from  a  Piiotograph  by  Prof.  Mayer). 


584  FRAGMENTS   OF  SCIENCE 

reason  upon  them,  and,  by  determined  study,  attain  to 
clearness  of  conception.  Facts  thus  dealt  with  exercise 
an  expansive  force  upon  the  intellect — they  widen  the 
mind  to  generalization.  We  soon  recognize  a  brother- 
hood between  the  larger  phenomena  of  Nature  and  the 
minute  effects  which  we  have  observed  in  our  private 
chambers.  Why,  we  inquire,  does  the  magnetic  needle 
set  north  and  south?  Evidently  it  is  compelled  to  do 
so  by  the  earth;  the  great  globe  which  we  inherit  is  itself 
a  magnet. 

Let  us  learn  a  little  more  about  it.  By  means  of  a 
bit  of  wax,  or  otherwise,  attach  the  end  of  your  silk 
fibre  to  the  middle  point  of  your  magnetic  needle;  the 
needle  will  thus  be  uninterfered  with  by  the  paper  loop, 
and  will  enjoy  to  some  extent  a  power  of  * 'dipping"  its 
point,  or  its  eye,  below  the  horizon.  Lay  your  bar-magnet 
on  a  table,  and  hold  the  needle  over  the  equator  of  the 
magnet.  The  needle  sets  horizontal.  Move  it  toward  the 
north  end  of  the  magnet;  the  south  end  of  the  needle 
dips,  the  dip  augmenting  as  you  approach  the  north  pole, 
over  which  the  needle,  if  free  to  move,  will  set  itself  ex- 
actly vertical.  Move  it  back  to  the  centre,  it  resumes  its 
horizon tality;  pass  it  on  toward  the  south  pole,  its  north 
end  now  dips,  and  directly  over  the  south  pole  the  needle 
becomes  vertical,  its  north  end  being  now  turned  down- 
ward. Thus  we  learn  that  on  the  one  side  of  the  mag- 
netic equator  the  north  end  of  the  needle  dips;  on  the 
other  side  the  south  end  dips,  the  dip  varjdng  from 
nothing  to  90°.  If  we  go  to  the  equatorial  regions  of  the 
earth  with  a  suitably  suspended  needle  we  shall  find  there 
the  position  of  the  needle  horizontal.  If  we  sail  north  one 
end  of  the  needle  dips;   if  we  sail  south  the  opposite  end 


ELEMENTARY  MAGNETISM  885 

dips;  and  over  the  north  or  south  terrestrial  magnetic 
pole  the  needle  sets  vertical.  The  south  magnetic  pole 
has  not  yet  been  found,  but  Sir  James  Boss  discovered 
the  north  magnetic  pole  on  June  1,  1831.  In  this  manner 
we  establish  a  complete  parallelism  between  the  action  of 
the  earth  and  that  of  an  ordinary  magnet. 

The  terrestrial  magnetic  poles  do  not  coincide  with  the 
geographical  ones;  nor  does  the  earth's  magnetic  equator 
quite  coincide  with,  the  geographical  equator.  The  direc- 
tion of  the  magnetic  needle  in  London,  which  is  called  the 
magnetic  meridian,  encloses  an  angle  of  24°  with  the  as- 
tronomical meridian,  this  angle  being  called  the  Declina- 
tion of  the  needle  for  London.  The  north  pole  of  the 
needle  now  lies  to  the  west  of  the  true  meridian ;  the  dec- 
lination is  westerly.  In  the  year  1660,  however,  the  decli- 
nation was  nothing,  while  before  that  time  it  was  easterly. 
All  this  proves  that  the  earth's  magnetic  constituents  are 
gradually  changing  their  distribution.  This  change  is 
very  slow:  it  is  therefore  called  the  secular  change^ 
and  the  observation  cf  it  has  not  yet  extended  over  a 
sufficient  period  to  enable  us  to  guess,  even  approxi- 
mately, at  its  laws. 

Having  thus  discovered,  to  some  extent,  the  secret  of 
the  earth's  magnetic  power,  we  can  turn  it  to  account. 
In  the  line  of  "dip"  I  hold  a  poker  formed  of  good  soft 
iron.  The  earth,  acting  as  a  magnet,  is  at  this  moment 
constraining  the  two  fluids  of  the  poker  to  separate,  mak- 
ing the  lower  end  of  the  poker  a  north  pole,  and  the 
upper  end  a  south  pole.  Mark  the  experiment:  When 
the  knob  is  uppermost,  it  attracts  the  north  end  of  a  mag- 
netic needle;  when  undermost  it  attracts  the  south  end  of 
a  magnetic  needle.      With   such  a  poker  repeat  this  ex- 

SOIENCE 11 


386  FRAGMENTS   OF  SCIENCE 

periment  and  satisfy  yourselves  that  the  fluids  shift  their 
position  according  to  the  manner  in  which  the  poker  is 
presented  to  the  earth.  It  has  already  been  stated  that 
the  softest  iron  possesses  a  certain  amount  of  coercive 
force.  The  earth,  at  this  moment,  finds  in  this  force 
an  antagonist  which  opposes  the  decomposition  of  the 
neutral   fluid. 

The  component  fluids  may  be  figured  as  meeting  an 
amount  of  friction,  or  possessing  an  amount  of  adhesion, 
which  prevents  them  from  gliding  over  the  molecules  of 
the  poker.  Can  we  assist  the  earth  in  this  case?  If  we 
wish  to  remove  the  residue  of  a  powder  from  the  interior 
surface  of  a  glass  to  which  the  powder  clings,  we  invert 
the  glass,  tap  it,  loosen  the  hold  of  the  powder,  and  thus 
enable  the  force  of  gravity  to  pull  it  down.  So  also  by 
tapping  the  end  of  the  poker  we  loosen  the  adhesion  of 
the  magnetic  fluids  to  the  molecules  and  enable  the  earth 
to  pull  them  apart.  But  what  is  the  consequence?  The 
portion  of  fluid  which  has  been  thus  forcibly  dragged  over 
the  molecules  refuses  to  return  when  the  poker  has  been 
removed  from  the  line  of  dip;  the  iron,  as  you  see,  has 
become  a  permanent  magnet.  By  reversing  its  position 
and  tapping  it  again  we  reverse  its  magnetism.  A  thought- 
ful and  competent  teacher  will  know  how  to  place  these 
remarkable  facts  before  his  pupils  in  a  manner  which  will 
excite  their  interest.  By  the  use  of  sensible  images,  more 
or  less  gross,  he  will  first  give  those  whom  he  teaches  defi- 
nite conceptions,  purifying  these  conceptions  afterward,  as 
the  minds  of  his  pupils  become  more  capable  of  abstrac- 
tion. By  thus  giving  them  a  distinct  substratum  for  their 
reasonings,  he  will  confer  upon  his  pupils  a  profit  and  a 
joy  which  the  mere  exhibition  of  facts  without  principles, 


ELEMENTARY  MAGNETISM  387 

or   the    appeal    to    the    bodily   senses   and   the   power   of 
memory  alone,    could   never  inspire. 


As  an  expansion  of  the  note  at  p.  37 7,  the  following  extract  may  find  a 
place  here : 

**It  is  well  known  that  a  voltaic  current  exerts  an  attractive  force  upon 
a  second  current,  flowing  in  the  same  direction ;  and  that  when  the  directions 
are  opposed  to  each  other  the  force  exerted  is  a  repulsive  one.  By  coiling  wires 
into  spirals,  Ampere  was  enabled  to  make  them  produce  all  the  phenomena  of 
attraction  and  repulsion  exhibited  by  magnets,  and  from  this  it  was  but  a  step 
to  his  celebrated  theory  of  molecular  currents.  He  supposed  the  molecules  of 
a  magnetic  body  to  be  surrounded  by  such  currents,  which,  however,  in  the 
natural  state  of  the  body  mutually  neutralized  each  other,  on  account  of  their 
confused  grouping.  The  act  of  magnetization  he  supposed  to  consist  in  setting 
these  molecular  currents  parallel  to  each  other ;  and,  starting  from  this  principle, 
he  reduced  all  the  phenomena  of  magnetism  to  the  mutual  action  of  electric 
currents. 

*'If  we  reflect  upon  the  experiments  recorded  in  the  foregoing  pages  from 
first  to  last,  we  can  hardly  fail  to  be  convinced  that  diamagnetic  bodies  oper- 
ated on  by  magnetic  forces  possess  a  polarity  'the  same  in  kind  as,  but  the 
reverse  in  direction  of,  that  acquired  by  magnetic  bodies. '  But  if  this  be  the 
case,  how  are  we  to  conceive  the  physical  mechanism  of  this  polarity?  Accord- 
ing to  Coulomb's  and  Poisson's  theory,  the  act  of  magnetization  consists  in  the 
decomposition  of  a  neutral  magnetic  fluid ;  the  north  pole  of  a  magnet,  for  ex- 
ample, possesses  an  attraction  for  the  south  fluid  of  a  piece  of  soft  iron  sub- 
mitted to  its  influence,  draws  the  said  fluid  toward  it,  and  with  it  the  material 
particles  with  which  the  fluid  is  associated.  To  account  for  diamagnetic  phe- 
nomena this  theory  seems  to  fail  altogether ;  according  to  it,  indeed,  the  oft-used 
phrase,  'a  north  pole  exciting  a  north  pole,  and  a  south  pole  a  south  pole,'  in- 
volves a  contradiction.  For  if  the  north  fluid  be  supposed  to  be  attracted 
toward  the  influencing  north  pole,  it  is  absurd  to  suppose  that  its  presence  there 
could  produce  repulsion.  The  theory  of  Ampere  is  equally  at  a  loss  to  explain 
diamagnetic  action ;  for  if  we  suppose  the  particles  of  bismuth  surrounded  by 
molecular  currents,  then,  according  to  all  that  is  known  of  electro-dynamic 
laws,  these  currents  would  set  themselves  parallel  to,  and  in  the  same  direction 
as,  those  of  the  magnet,  and  hence  attraction,  and  not  repulsion,  would  be  the 
result.  The  fact,  however,  of  this  not  being  the  case,  proves  that  these  molec- 
ular currents  are  not  the  mechanism  by  which  diamagnetic  induction  is  effected. 
The  consciousness  of  this,  I  doubt  not,  drove  M.  "Weber  to  the  assumption  that 
the  phenomena  of  diamagnetism  are  produced  by  molecular  currents,  not  directed, 
but  actually  excited  in  the  bismuth  by  the  magnet.     Such  induced  currents 


S88  FRAGMENTS   OF  SCIENCE 

would,  according  to  known  laws,  have  a  direction  opposed  to  those  of  the 
inducing  magnet,  and  hence  would  produce  the  phenomena  of  repulsion.  To 
carry  out  the  assumption  here  made,  M.  Weber  is  obliged  to  suppose  that  the 
molecules  of  diamagnetic  bodies  are  surrounded  by  channels,  in  which  the  in- 
duced molecular  currents,  once  excited,  continue  to  flow  without  resistance."* 
*'Diamagnetism  and  Magne-crystallic  Action,"  pp.  ISS-ISY. 


'  In  assuming  these  non-resisting  channels  M.  Weber,  it  must  be  admitted, 
did  not  go  beyond  the  assumptions  of  Ampere. 


XVI 

ON   FOECE* 

A  SPHERE  of  lead  was  suspended  at  a  height  of  16 
feet  above  the  theatre  floor  of  the  Royal  Institu- 
tion. It  was  liberated,  and  fell  by  gravity.  That 
weight  required  a  second  to  fall  to  the  floor  from  that 
elevation;  and  the  instant  before  it  touched  the  floor  it 
had  a  velocity  of  82  feet  a  second.  That  is  to  say,  if  at 
that  instant  the  earth  were  annihilated,  and  its  attraction 
annulled,  the  weight  would  proceed  through  space  at  the 
uniform  velocity  of  82"^  feet  a  second. 

If  instead  of  being  pulled  downward  by  gravity,  the 
weight  be  cast  upward  in  opposition  to  gravity,  then,  to 
reach  a  height  of  16  feet  it  must  start  with  a  velocity  of 
82  feet  a  second.  This  velocity  imparted  to  the  weight 
by  the  human  hand,  or  by  any  other  mechanical  means, 
would  carry  it  to  the  precise  height  from  which  we  saw 
it  fall. 

Now  the  lifting  of  the  weight  may  be  regarded  as  so 
much  mechanical  work  performed.  By  means  of  a  ladder 
placed  against  the  wall,  the  weight  might  be  carried  up 
to  a  height  of  16  feet;  or  it  might  be  drawn  up  to  this 
height  by  means  of  a  string  and  pulley,  or  it  might  be 
suddenly  jerked  up  to  a  height  of  16  feet.     The  amount 

*  A  discourse  delivered  in  the  Royal  Institution,  June  6,  1862. 

(889) 


890  FRAGMENTS   OF  SCIENCE 

of  work  done  in  all  these  cases,  as  far  as  the  raising  of 
the  weight  is  concerned,  would  be  absolutely  the  same. 
The  work  done  at  one  and  the  same  place,  and  neglecting 
the  small  change  of  gravity  with  the  height,  depends 
solely  upon  two  things;  on  the  quantity  of  matter  lifted, 
and  on  the  height  to  which  it  is  lifted.  If  we  call  the 
quantity  or  mass  of  matter  m,  and  the  height  through 
which  it  is  lifted  h,  then  the  product  of  m  into  ^,  or  m  A, 
expresses,  or  is  proportional  to,  the  amount  of  work  done. 

Supposing,  instead  of  imparting  a  velocity  of  82  feet 
a  second  we  impart  at  starting  twice  this  velocity.  To 
what  height  will  the  weight  rise?  You  might  be  disposed 
to  answer,  *'To  twice  the  height";  but  this  would  be 
quite  incorrect.  Instead  of  twice  16,  or  82  feet,  it  would 
reach  a  height  of  four  times  16,  or  64  feet.  So  also,  if 
we  treble  the  starting  velocity,  the  weight  would  reach 
nine  times  the  height;  if  we  quadruple  the  speed  at  start- 
ing, we  attain  sixteen  times  the  height.  Thus,  with  a 
fourfold  velocity  of  128  feet  a  second  at  starting,  the 
weight  would  attain  an  elevation  of  256  feet.  With  a 
sevenfold  velocity  at  starting,  the  weight  would  rise  to  49 
times  the  height,  or  to  an  elevation  of  784  feet. 

Now  the  work  done — or,  as  it  is  sometimes  called,  the 
mechanical  effect — other  things  being  constant,  is,  as  be- 
fore explained,  proportional  to  the  height,  and  as  a  double 
velocity  gives  four  times  the  height,  a  treble  velocity  nine 
times  the  height,  and  so  on,  it  is  perfectly  plain  that  the 
mechanical  effect  increases  as  the  square  of  the  velocity. 
If  the  mass  of  the  body  be  represented  by  the  letter  m, 
and  its  velocity  by  v,  the  mechanical  effect  would  be  pro- 
portional to  or  represented  by  m  v^.  In  the  case  consid- 
ered, I  have  supposed  the  weight  to  be  cast  upward,  bei&^ 


ON  FORCE  891 

opposed  in  its  flight  by  ttie  resistance  of  gravity;  but  the 
same  holds  true  if  the  projectile  be  sent  into  water,  mud, 
earth,  timber,  or  other  resisting  material.  If,  for  example, 
we  double  the  velocity  of  a  cannon-ball,  we  quadruple  its 
mechanical  effect.  Hence  the  importance  of  augmenting 
the  velocity  of  a  projectile,  and  hence  the  philosophy  of 
Sir  William  Armstrong  in  using  a  large  charge  of  powder 
in  his  recent  striking  experiments. 

The  measure  then  of  mechanical  effect  is  the  mass  of 
the  body  multiplied  by  the  square  of  its  velocity. 

Now  in  firing  a  ball  against  a  target  the  projectile, 
after  collision,  is  often  found  hot.  Mr.  Fairbairn  informs 
me  that  in  the  experiments  at  Shoeburyness  it  is  a  com- 
mon thing  to  see  a  flash,  even  in  broad  daylight,  when  the 
ball  strikes  the  target.  And  if  our  lead  weight  be  exam- 
ined after  it  has  fallen  from  a  height  it  is  also  found 
heated.  Now  here  experiment  and  reasoning  lead  us  to 
the  remarkable  law  that,  like  the  mechanical  effect,  the 
amount  of  heat  generated  is  proportional  to  the  product 
of  the  mass  into  the  square  of  the  velocity.  Double  your 
mass,  other  things  being  equal,  and  you  double  your 
amount  of  heat;  double  your  velocity,  other  things  re- 
maining equal,  and  you  quadruple  your  amount  of  heat. 
Here  then  we  have  common  mechanical  motion  destroyed 
and  heat  produced.  When  a  violin  bow  is  drawn  across 
a  string,  the  sound  produced  is  due  to  motion  imparted 
to  the  air,  and  to  produce  that  motion  muscular  force  has 
been  expended.  We  may  here  correctly  say  that  the  me- 
chanical force  of  the  arm  is  converted  into  music.  In  a 
similar  way  we  say  that  the  arrested  motion  of  our  de- 
scending weight,  or  of  the  cannon-ball,  is  converted  into 
heat.     The  mode  of  motion  changes,  but  motion  still  con- 


892  FRAGMENTS    OF   SCIENCE 

tinues;  tlie  motion  of  the  mass  is  converted  into  a  motion 
of  tlie  atoms  of  the  mass;  and  these  small  motions,  com- 
municated to  the  nerves,  produce  the  sensation  we  call 
heat. 

We  know  the  amount  of  heat  which  a  given  amount  of 
mechanical  force  can  develop.  Our  lead  ball,  for  exam- 
ple, in  falling  to  the  earth  generated  a  quantity  of  heat 
sufficient  to  raise  its  own  tempera t»re  three- fifths  of  a 
Fahrenheit  degree.  It  reached  the  earth  with  a  velocity 
of  32  feet  a  second,  and  forty  times  this  velocity  would  be 
small  for  a  rifle  bullet;  multiplying  gths  by  the  square 
of  40,  we  find  that  the  amount  of  heat  developed  by  col- 
lision with  the  target  would,  if  wholly  concentrated  in  the 
lead,  raise  its  temperature  960  degrees.  This  would  be 
more  than  sufficient  to  fuse  the  lead.  In  reality,  how- 
ever, the  heat  developed  is  divided  between  the  lead  and 
the  body  against  which  it  strikes;  nevertheless,  it  would 
be  worth  while  to  pay  attention  to  this  point,  and  to  as- 
certain whether  rifle  bullets  do  not,  under  some  circum- 
stances, show  signs  of  fusion.  ^ 

From  the  motion  of  sensible  masses,  by  gravity  and 
other  means,  we  now  pass  to  the  motion  of  atoms  toward 
each  other  by  chemical  affinity.  A  collodion  balloon  filled 
with  a  mixture  of  chlorine  and  hydrogen  being  hung  in 
the  focus  of  a  parabolic  mirror,  in  the  focus  of  a  second 
mirror  20  feet  distant  a  strong  electric  light  was  suddenly 
generated;  the  instant  the  concentrated  light  fell  upon  the 
balloon,    the  gases  within  it    exploded,   hydrochloric   acid 


*  Eight  years  subsequently  this  surmise  was  proved  correct.  In  the  Franco- 
German  War  signs  of  fusion  were  observed  in  the  case  of  bullets  impinging  on 
bones. 


ON  FORCE  393 

being  the  result.  Here  the  atoms  virtually  fell  together, 
the  amount  of  heat  produced  showing  the  enormous  force 
of  the  collision.  The  burning  of  charcoal  in  oxygen  is 
an  old  experiment,  but  it  has  now  a  significance  beyond 
what  it  used  to  have;  we  now  regard  the  act  of  combina- 
tion on  the  part  of  the  atoms  of  oxygen  and  coal  as  we 
regard  the  clashing  of  a  falling  weight  against  the  earth. 
The  heat  produced  in  both  cases  is  referrible  to  a  common 
cause.  A  diamond,  which  burns  in  oxygen  as  a  star  of 
white  light,  glows  and  burns  in  consequence  of  the  falling 
of  the  atoms  of  oxygen  against  it.  And  could  we  meas- 
ure the  velocity  of  the  atoms  when  they  clash,  and  could 
we  find  their  number  and  weights,  multiplying  the  weight 
of  each  atom  by  the  square  of  its  velocity,  and  adding  all 
together,  we  should  get  a  number  representing  the  exact 
amount  of  heat  developed  by  the  union  of  the  oxygen  and 
carbon. 

Thus  far  we  have  regarded  the  heat  developed  by  the 
clashing  of  sensible  masses  and  of  atoms.  Work  is  ex- 
pended in  giving  motion  to  these  atoms  or  masses,  and 
heat  is  developed.  But  we  reverse  this  process  daily,  and 
by  the  expenditure  of  heat  execute  work.  We  can  raise 
a  weight  by  heat;  and  in  this  agent  we  possess  an  enor- 
mous store  of  mechanical  power.  A  pound  of  coal  produces 
by  its  combination  with  oxygen  an  amount  of  heat  which, 
if  mechanically  applied,  would  suffice  to  raise  a  weight 
of  100  lbs.  to  a  height  of  20  miles  above  the  earth's  sur- 
face. Conversely,  100  lbs.  falling  from  a  height  of  20 
miles,  and  striking  against  the  earth,  would  generate  an 
amount  of  heat  equal  to  that"  developed  by  the  combus- 
tion of  a  pound  of  coal.  Wherever  work  is  done  by  heat, 
heat  disappears.     A  gun  which  fires  a  ball  is  less  heated 


894  FRAGMENTS   OF  SCIENCE 

than  one  whicli  fires  a  blank  cartridge.  The  quantity  of 
lieat  communicated  to  the  boiler  of  a  working  steam- 
engine  is  greater  than  that  which  could  be  obtained  from 
the  recondensation  of  the  steam,  after  it  had  done  its 
work;  and  the  amount  of  work  performed  is  the  exact 
equivalent  of  the  amount  of  heat  lost.  Mr.  Smyth  in- 
formed us,  in  his  interesting  discourse,  that  we  dig  an- 
nually 84  millions  of  tons  of  coal  from  our  pits.  The 
amount  of  mechanical  force  represented  by  this  quantity 
of  coal  seems  perfectly  fabulous.  The  combustion  of 
a  single  pound  of  coal,  supposing  it  to  take  place  in  a 
minute,  would  be  equivalent  to  the  work  of  800  horses; 
and  if  we  suppose  108  millions  of  horses  working  day  and 
night  with  unimpaired  strength,  for  a  year,  their  united 
energies  would  enable  them  to  perform  an  amount  of  work 
just  equivalent  to  that  which  the  annual  produce  of  our 
coal-fields  would  be  able  to  accomplish. 

Comparing  with  ordinary  gravity  the  force  with  which 
oxygen  and  carbon  unite  together,  chemical  affinity  seems 
almost  infinite.  But  let  us  give  gravity  fair  play  by  per- 
mitting it  to  act  throughout  its  entire  range.  Place  a 
body  at  such  a  distance  from  the  earth  that  the  attraction 
of  our  planet  is  barely  sensible,  and  let  it  fall  to  the  earth 
from  this  distance.  It  would  reach  the  earth  with  a  final 
velocity  of  ^Q^  747  feet  a  second ;  and  on  collision  with  the 
earth  the  body  would  generate  about  twice  the  amount  of 
heat  generated  by  the  combustion  of  an  equal  weight 
of  coal.  We  have  stated  that  by  falling  through  a  space 
of  16  feet  our  lead  bullet  would  be  heated  three- fifths  of 
a  degree;  but  a  body  falling  from  an  infinite  distance  has 
already  used  up  1,299,999  parts  out  of  1,800,000  of  the 
earth's  pulling  power,  when  it  has  arrived  within  16  feet 


ON  FORCE  895 

of   the   surface;    on  this  space  only  i,3oo,oooths  of   the  whole 
force  is  exerted. 

Let  us  now  turn  our  thoughts  for  a  moment  from  the 
earth  to  the  sun.  The  researches  of  Sir  John  Herschel 
and  M.  Pouillet  have  informed  us  of  the  annual  expendi- 
ture of  the  sun  as  regards  heat;  and  by  an  easy  calcula- 
tion we  ascertain  the  precise  amount  of  the  expenditure 
which  falls  to  the  share  of  our  planet.  Out  of  2,300  million 
parts  of  light  and  heat  the  earth  receives  one.  The  whole 
heat  emitted  by  the  sun  in  a  minute  would  be  competent 
to  boil  12,000  millions  of  cubic  miles  of  ice-cold  water. 
How  is  this  enormous  loss  made  good — whence  is  the 
sun's  heat  derived,  and  by  what  means  is  it  maintained? 
No  combustion — no  chemical  affinity  with  which  we  are 
acquainted — would  be  competent  to  produce  the  tempera- 
ture of  the  sun's  surface.  Besides,  were  the  sun  a  burn- 
ing body  merely,  its  light  and  heat  would  speedily  come 
to  an  end.  Supposing  it  to  be  a  solid  glbbe  of  coal,  its 
combustion  would  only  cover  4,600  years  of  expenditure. 
In  this  short  time  it  would  burn  itself  out.  What  agency 
then  can  produce  the  temperature  and  maintain  the  out- 
lay? We  have  already  regarded  the  case  of  a  body  fall- 
ing from  a  great  distance  toward  the  earth,  and  found 
that  the  heat  generated  by  its  collision  would  be  twice 
that  produced  by  the  combustion  of  an  equal  weight  of 
coal.  How  much  greater  must  be  the  heat  developed  by 
a  body  falling  against  the  sun!  The  maximum  velocity 
with  which  a  body  can  strike  the  earth  is  about  7  miles 
in  a  second;  the  maximum  velocity  with  which  it  can 
strike  the  sun  is  390  miles  in  a  second.  And  as  the  heat 
developed  by  the  collision  is  proportional  to  the  square 
of  the  velocity  destroyed,  an  asteroid  falling  into  the  sun 


FRAGMENTS    OF  SCIENCE 

with  the  above  velocity  would  generate  about  10,000  times 
the  quantity  of  heat  produced  by  the  combustion  of  an 
asteroid  of  coal  of  the  same  weight. 

Have  we  any  reason  to  believe  that  such  bodies  exist 
in  space,  and  that  they  may  be  raining  down  upon  the 
sun?  The  meteorites  flashing  through  the  air  are  small 
planetary  bodies,  drawn  by  the  earth's  attraction.  They 
enter  our  atmosphere  with  planetary  velocity,  and  by  fric- 
tion against  the  air  they  are  raised  to  incandescence  and 
caused  to  emit  light  and  heat.  At  certain  seasons  of  the 
year  they  shower  down  upon  us  in  great  numbers.  In 
Boston  240,000  of  them  were  observed  in  nine  hours. 
There  is  no  reason  to  suppose  that  the  planetary  system 
is  limited  to  "vast  masses  of  enormous  weight";  there  is, 
on  the  contrary,  reason  to  believe  that  space  is  stocked 
with  smaller  masses,  which  obey  the  same  laws  as  the 
larger  ones.  That  lenticular  envelope  which  surrounds 
the  sun,  and  which  is  known  to  astronomers  as  the  Zodi- 
acal light,  is  probably  a  crowd  of  meteors;  and,  moving 
as  they  do  in  a  resisting  medium,  they  must  continually 
approach  the  sun.  Falling  into  it,  they  would  produce 
enormous  heat,  and  this  would  constitute  a  source  from 
which  the  annual  loss  of  heat  might  be  made  good.  The 
sun,  according  to  this  hypothesis,  would  continually  grow 
larger;  but  how  much  larger?  "Were  our  moon  to  fall 
into  the  sun,  it  would  develop  an  amount  of  heat  suffi- 
cient to  cover  one  or  two  years'  loss;  and  were  our  earth 
to  fall  into  the  sun  a  century's  loss  would  be  made  good. 
Still,  our  moon  and  our  earth,  if  distributed  over  the  sur- 
face of  the  sun,  would  utterly  vanish  from  perception. 
Indeed,  the  quantity  of  matter  competent  to  produce  the 
required  effect  would,  during  the  range  of  history,  cause 


ON  FORCE  397 

no  appreciable  augmentation  in  the  sun's  magnitude.  The 
augmentation  of  the  sun's  attractive  force  would  be  more 
sensible.  However  this  hypothesis  may  fare  as  a  repre- 
sentant  of  what  is  going  on  in  Nature,  it  certainly  shows 
how  a  sun  might  be  formed  and  maintained  on  known 
thermo-dynamic  principles. 

Our  earth  moves  in  its  orbit  with  a  velocity  of  68,040 
miles  an  hour.  Were  this  motion  stopped,  an  amount  of 
heat  would  be  developed  sufficient  to  raise  the  tempera- 
ture of  a  globe  of  lead  of  the  same  size  as  the  earth 
884,000  degrees  of  the  centigrade  thermometer.  It  has 
been  prophesied  that  '*the  elements  shall  melt  with  fer- 
vent heat."  The  earth's  own  motion  embraces  the  con- 
ditions of  fulfilment;  stop  that  motion,  and  the  greater 
part,  if  not  the  whole,  of  our  planet  would  be  reduced 
to  vapor.  If  the  earth  fell  into  the  sun,  the  amount  of 
heat  developed  by  the  shock  would  be  equal  to  that  de- 
veloped by  the  combustion  of  a  mass  of  solid  coal  6,485 
times  the  earth  in  size. 

There  is  one  other  consideration  connected  with  the 
permanence  of  our  present  terrestrial  conditions  which  is 
well  worthy  of  our  attention.  Standing  upon  one  of  the 
London  bridges,  we  observe  the  current  of  the  Thames  re- 
versed, and  the  water  poured  upward  twice  a  day.  The 
water  thus  moved  rubs  against  the  river's  bed,  and  heat 
is  the  consequence  of  this  friction.  The  heat  thus  gen- 
erated is  in  part  radiated  into  space  anJ  lost,  as  far  as 
the  earth  is  concerned.  What  supplies  this  incessant  loss  ? 
The  earth's  rotation.  Let  us  look  a  little  more  closely  at 
the  matter.  Imagine  the  moon  fixed,  and  the  earth  turn- 
ing like  a  wheel  from  west  to  east  in  its  diurnal  rotation. 
Suppose  a  high  mountain  on  the  earth's  surface  approach- 


898  FRAGMENTS   OF  SCIENCE 

ing  the  earth's  meridian;  that  mountain  is,  as  it  were,  laid 
hold  of  by  the  moon ;  it  forms  a  kind  of  handle  by  which 
the  earth  is  pulled  more  quickly  round.  But  when  the 
meridian  is  passed  the  pull  of  the  moon  on  the  mountain 
would  be  in  the  opposite  direction,  it  would  tend  to  di- 
minish the  velocity  of  rotation  as  much  as  it  previously 
augmented  it;  thus  the  action  of  all  fixed  bodies  on  the 
earth's  surface  is  neutralized.  But  suppose  the  mountain 
to  lie  always  to  the  east  of  the  moon's  meridian,  the  pull 
then  would  be  always  exerted  against  the  earth's  rotation, 
the  velocity  of  which  would  be  diminished  in  a  degree 
corresponding  to  the  strength  of  the  pull.  The  tidal  wave 
occupies  this  position — it  lies  always  to  the  east  of  the 
moon's  meridian.  The  waters  of  the  ocean  are  in  part 
dragged  as  a  brake  along  the  surface  of  the  earth;  and  as 
a  brake  they  must  diminish  the  velocity  of  the  earth's 
rotation.^  Supposing  then  that  we  turn  a  mill  by  the  ac- 
tion of  the  tide,  and  produce  heat  by  the  friction  of  the 
millstones;  that  heat  has  an  origin  totally  different  from 
the  heat  produced  by  another  mill  which  is  turned  by  a 
mountain  stream.  The  former  is  produced  at  the  expense 
of  the  earth's  rotation,  the  latter  at  the  expense  of  the 
sun's  radiation. 

The  sun,  by  the  act  of  vaporization,  lifts  mechanically 
all  the  moisture  of  our  air,  which,  when  it  condenses,  falls 
in  the  form  of  rain,  and  when  it  freezes  falls  as  snow.  In 
this  solid  form  it  is  piled  upon  the  Alpine  heights,  and 
furnishes  materials  for  glaciers.  But  the  sun  again  inter- 
poses,  liberates  the  solidified  liquid,  and  permits  it  to  roll 
by  gravity  to  the   sea.      The  mechanical  force  of  every 

*  Kant  surmised  an  action  of  this  kind. 


ON   FORCE  399 

river  in  the  world  as  it  rolls  toward  the  ocean  is  drawn 
from  the  heat  of  the  sun.  No  streamlet  glides  to  a  lower 
level  without  having  been  first  lifted  to  the  elevation  from 
which  it  springs  by  the  power  of  the  sun.  The  energy 
of  winds  is  also  due  entirely  to  the  same  power. 

But  there  is  still  another  work  which  the  sun  performs, 
and  its  connection  with  which  is  not  so  obvious.  Trees 
and  vegetables  grow  upon  the  earth,  and  when  burned 
they  give  rise  to  heat,  and  hence  to  mechanical  energy. 
Whence  is  this  power  derived?  You  see  this  oxide  of 
iron,  produced  by  the  falling  together  of  the  atoms  of  iron 
and  oxygen;  you  cannot  see  this  transparent  carbonic  acid 
gas,  formed  by  the  falling  together  of  carbon  and  oxygen. 
The  atoms  thus  in  close  union  resemble  our  lead  weight 
while  resting  on  the  earth;  but  we  can  wind  up  the 
weight  and  prepare  it  for  another  fall,  and  so  these  atoms 
can  be  wound  up  and  thus  enabled  to  repeat  the  process 
of  combination.  In  the  building  of  plants  carbonic  acid 
is  the  material  from  which  the  carbon  of  the  plant  is  de- 
rived; and  the  solar  beam  is  the  agent  which  tears  the 
atoms  asunder,  setting  the  oxygen  free,  and  allowing  the 
carbon  to  aggregate  in  woody  fibre.  Let  the  solar  rays 
fall  upon  a  surface  of  sand;  the  sand  is  heated,  and 
finally  radiates  away  as  much  heat  as  it  receives;  let  the 
same  beams  fall  upon  a  forest,  the  quantity  of  heat  given 
back  is  less  than  the  forest  receives;  for  the  energy  of  a 
portion  of  the  sunbeams  is  invested  in  building  the  trees. 
"Without  the  sun  the  reduction  of  the  carbonic  acid  can- 
not be  effected,  and  an  amount  of  sunlight  is  consumed 
exactly  equivalent  to  the  molecular  work  done.  Thus 
trees  are  formed;  thus  the  cotton  on  which  Mr.  Bazley 
discoursed  last  Friday  is  produced.     I  ignite  this  cotton, 


400  FRAGMENTS   OF  SCIENCE 

and  it  flames;  the  oxygen  again  unites  with  the  carbon; 
but  an  amount  of  heat  equal  to  that  produced  by  its 
combustion  was  sacrificed  by  the  sun  to  form  that  bit 
of  cotton. 

We  cannot,  however,  stop  at  vegetable  life,  for  it  is 
the  source,  mediate  or  immediate,  of  all  animal  life.  The 
sun  severs  the  carbon  from  its  oxygen  and  builds  the  veg- 
etable; the  animal  consumes  the  vegetable  thus  formed, 
a  reunion  of  the  several  elements  takes  place,  producing 
animal  heat.  The  process  of  building  a  vegetable  is  one 
of  winding  up;  the  process  of  building  an  animal  is  one 
of  running  down.  The  warmth  of  our  bodies,  and  every 
mechanical  energy  which  we  exert,  trace  their  lineage  di- 
rectly to  the  sun.  The  fight  of  a  pair  of  pugilists,  the 
motion  of  an  army,  or  the  lifting  of  his  own  body  by 
an  Alpine  climber  up  a  mountain  slope,  are  all  cases  of 
mechanical  energy  drawn  from  the  sun.  A  man  weighing 
150  pounds  has  64  pounds  of  muscle;  but  these,  when 
dried,  reduce  themselves  to  15  pounds.  Doing  an  ordi- 
nary day's  work,  for  eighty  days,  this  mass  of  muscle 
would  be  wholly  oxidized.  Special  organs  which  do  more 
work  would  be  more  quickly  consumed:  the  heart,  for  ex- 
ample, if  entirely  unsustained,  would  be  oxidized  in  about 
a  week.  Take  the  amount  of  heat  due  to  the  direct  oxi- 
dation of  a  given  weight  of  food;  less  heat  is  developed 
by  the  oxidation  of  the  same  amount  of  food  in  the  work- 
ing animal  frame,  and  the  missing  quantity  is  the  equiva- 
lent of  the  mechanical  work  accomplished  by  the  muscles. 

I  might  extend  these  considerations;  the  work,  indeed, 
is  done  to  my  hand — but  I  arn  warned  that  you  have  been 
already  kept  too  long.  To  whom  then  are  we  indebted 
for    the    most    striking    generalizations   of    this    evening's 


ON  FORCE  401 

discourse  ?  They  are  the  work  of  a  man  ol  whom  you 
have  scarcely  ever  heard — the  published  labors  of  a  Ger- 
man doctor,  named  May  en  Without  external  stimulus, 
and  pursuing  his  profession  as  town  physician  in  Heil- 
bronn,  this  man  was  the  first  to  raise  the  conception  of 
the  interaction  of  heat  and  other  natural  forces  to  clear- 
ness in  his  own  mind.  And  yet  he  is  scarcely  ever  heard 
of,  and  even  to  scientific  men  his  merits  are  but  partially 
known.  Led  by  his  own  beautiful  researches,  and  quite 
independent  of  Mayer,  Mr.  Joule  published  in  1843  his 
first  paper  on  the  "Mechanical  Yalue  of  Heat'*;  but  in 
1842  Mayer  had  actually  calculated  the  mechanical  equiva- 
lent of  heat  from  data  which  ^only  a  man  of  the  rarest 
penetration  could  turn  to  account.  In  1845  he  published 
his  memoir  on  "Organic  Motion,"  and  applied  the  me- 
chanical theory  of  heat  in  the  most  fearless  and  precise 
manner  to  vital  processes.  He  also  embraced  the  other 
natural  agents  in  his  chain  of  conservation.  In  1853  Mr. 
"Waterston  proposed,  independently,  the  meteoric  theory 
of  the  sun's  heat,  and  in  1854  Professor  William  Thom- 
son applied  his  admirable  mathematical  powers  to  the 
development  of  the  theory;  but  six  years  previously  the 
subject  had  been  handled  in  a  masterly  manner  by  Mayer, 
and  all  that  I  have  said  about  it  has  been  derived  from 
him.  When  we  consider  the  circumstances  of  Mayer's 
life,  and  the  period  at  which  he  wrote,  we  cannot  fail  to 
be  struck  with  astonishment  at  what  he  has  accomplished. 
Here  was  a  man  of  genius  working  in  silence,  animated 
solely  by  a  love  of  his  subject,  and  arriving  at  the  most 
important  results  in  advance  of  those  whose  lives  were 
entirely  devoted  to  Natural  Philosophy.  It  was  the  acci- 
dent of  bleeding  a  feverish  patient  at  Java  in  1840  that 


402  FRAGMENTS    OF  SCIENCE 

led  Mayer  to  speculate  on  these  subjects.  He  noticed  that 
the  venous  blood  in  the  tropics  was  of  a  brighter  red  than 
in  colder  latitudes,  and  his  reasoning  on  this  fact  led  him 
into  the  laboratory  of  natural  forces,  where  he  has  worked 
with  such  signal  ability  and  success.  Well,  you  will  de- 
sire to  know  what  has  become  of  this  man.  His  mind, 
it  is  alleged,  gave  way;  it  is  said  he  became  insane,  and 
he  was  certainly  sent  to  a  lunatic  asylum.  In  a  biograph- 
ical dictionary  of  his  country  it  is  stated  that  he  died 
there,  but  this  is  incorrect.  He  recovered;  and,  I  believe, 
is  at  this  moment  a  cultivator  of  vineyards  in  Heilbronn. 


June  20,   1862. 

While  preparing  for  publication  my  last  course  of  lec- 
tures on  Heat,  I  wished  to  make  myself  acquainted  with 
all  that  Dr.  Mayer  had  done  in  connection  with  this  sub- 
ject. I  accordingly  wrote  to  two  gentlemen  who  above  all 
others  seemed  likely  to  give  me  the  information  which 
I  needed.*  Both  of  them  are  Germans,  and  both  par- 
ticularly distinguished  in  connection  with  the  Dynamical 
Theory  of  Heat.  Each  of  them  kindly  furnished  me  with 
the  list  of  Mayer's  publications,  and  one  of  them  (Clau- 
sius)  was  so  friendly  as  to  order  them  from  a  bookseller, 
and  to  send  them  to  me.  This  friend,  in  his  reply  to  my 
first  letter  regarding  Mayer,  stated  his  belief  that  I  should 
not  find  anything  very  important  in  Mayer's  writings; 
but  before  forwarding  the  memoirs  to  me  he  read  them 
himself.  His  letter  accompanying  them  contains  the  fol- 
lowing words:   "I  must  here  retract  the  statement  in  my 

^  Helmholtz  and  Clausius. 


ON  FORCE  403 

last  letter,  that  you  would  not  find  much  matter  of  impor- 
tance in  Mayer's  writings:  I  am  astonished  at  the  multi- 
tude of  beautiful  and  correct  thoughts  which  they  con- 
tain"; and  he  goes  on  to  point  out  various  important 
subjects,  in  the  treatment  of  which  Mayer  had  anticipated 
other  eminent  writers.  My  other  friend,  in  whose  own 
publications  the  name  of  Mayer  repeatedly  occurs,  and 
whose'  papers  containing  these  references  were  translated 
some  years  ago  by  myself,  was,  on  the  10th  of  last  month, 
unacquainted  with  the  thoughtful  and  beautiful  essay  of 
Mayer's,  entitled  "Beitrage  zur  Dynamik  des  Himmels," 
and  in  1854,  when  Professor  William  Thomson  developed 
in  so  striking  a  manner  the  meteoric  theory  of  the  sun*s 
heat,  he  was  certainly  not  aware  of  the  existence  of  that 
essay,  though  from  a  recent  article  in  "Macmillan's  Maga- 
zine" I  infer  that  he  is  now  aware  of  it.  Mayer's  physio- 
logical writings  have  been  referred  to  by  physiologists — 
by  Dr.  Carpenter,  for  example — in  terms  of  honoring 
recognition.  We  have  hitherto,  indeed,  obtained  frag- 
mentary glimpses  of  the  man,  partly  from  physicists  and 
partly  from  physiologists;  but  his  total  merit  has  never 
yet  been  recognized  as  it  assuredly  would  have  been  had 
he  chosen  a  happier  mode  of  publication.  I  do  not  think 
a  greater  disservice  could  be  done  to  a  man  of  science 
than  to  overstate  his  claims:  such  overstatement  is  sure 
to  recoil  to  the  disadvantage  of  him  in  whose  interest  it 
is  made.  But  when  Mayer's  opportunities,  achievements 
and  fate  are  taken  into  account,  I  do  not  think  that  I 
shall  be  deeply  blamed  for  attempting  to  place  him  in 
that  honorable  position  which  I  believe  to  be  his  due. 

Here,    however,    are   tlie   titles   of  Mayer's  papers,   the 
perusal  of  which  will  correct  any  error  of  judgment  into 


404  FRAGMENTS    OF  SCIENCE 

which  I  may  have  fallen  regarding  their  author.  "Bemer- 
kungen  Uber  die  Krafte  der  unbelebten  Natur,"  Liebig's 
"Annalen,"  1842,  vol.  42,  p.  231;  "Die  Organische  Be- 
wegung  in  ihrem  Zusammenhange  mit  dem  Stoffwechsel, " 
Heilbronn,  1845;  "Beitrage  zur  Dynamik  des  Himmels," 
Heilbronn,  1848;  "Bemerkungen  liber  das  Mechanische 
Equivalent  der  Warme,"  Heilbronn,  1851. 


In  Memoeiam. — Dr.  Julius  Eobert  Mayer  died  at  Heil- 
bronn on  March  20,  1878,  aged  63  years.  It  gives  me 
pleasure  to  reflect  that  the  great  position  which  he  will 
forever  occupy  in  the  annals  of  science  was  first  virtually 
assigned  to  him.  in  the  foregoing  discourse.  He  was  sub- 
sequently chosen  by  acclamation  a  member  of  the  French 
Academy  of  Sciences;  and  he  received  from  the  Eoyal 
Society  the  Copley  medal — its  highest  reward.' 


November,  1878. 
At  the  meeting  of  the  British  Association  at  Glasgow 
in  1876 — that  is  to  say,  more  than  fourteen  years  after  its 
delivery  and  publication — the  foregoing  lecture  was  made 
the  cloak  for  an  unseemly  personal  attack  by  Professor 
Tait.  The  anger  which  found  this  uncourteous  vent  dates 
from  1863,"  when  it  fell  to  my  lot  to  maintain,  in  opposi- 
tion to  him  and  a  more  eminent  colleague,  the  position 
which  in  1862  I  had  assigned  to  Dr.  Mayer.  In  those 
days  Professor  Tait  denied  to  Mayer  all  originality,  and 
he  has  since,  1  regret  to  say,  never  missed  an  opportunity, 

»  See  "The  Copley  Medalist  for  1871,"  p.  479. 

^  See  "Philosophical  Magazine'*  for  this  and  the  succeeding  years. 


ON   FORCE  405 

however  small,  of  carping  at  Mayer's  claims.  The  action 
of  the  Academy  of  Sciences  and  of  the  Royal  Society  sum- 
marily disposes  of  this  detraction,  to  which  its  object, 
during  his  lifetime,  never  vouchsafed  either  remonstrance 
or  reply. 

Some  time  ago  Professor  Tait  published  a  volume  of 
lectures  entitled  "Recent  Advances  in  Physical  Science," 
which  I  have  reason  to  know  has  evoked  an  amount  of 
censure  far  beyond  that  hitherto  publicly  expressed. 
Many  of  the  best  heads  on  the  continent  of  Europe  agree 
in  their  rejection  and  condemnation  of  the  historic  por- 
tions of  this  book.  In  March  last  it  was  subjected  to  a 
brief  but  pungent  critique  by  Du  Bois-Reymond,  the  cele- 
brated Perpetual  Secretary  of  the  Academy  of  Sciences  in 
Berlin.  Du  Bois-Reymond's  address  was  on  "National 
Feeling,"  and  his  critique  is  thus  wound  up:  "The  au- 
thor of  the  'Lectures'  is  not,  perhaps,  sufficiently  well 
acquainted  with  the  history  on  which  he  professes  to 
throw  light,  and  on  the  later  phases  of  which  he  passes 
so  unreserved  (schroff)  a  judgment.  He  thus  exposes 
himself  to  the  suspicion — which,  unhappily,  is  not  weak- 
ened by  his  other  writings — that  the  fiery  Celtic  blood  of 
his  country  occasionally  runs  away  with  him,  converting 
him  for  the  time  into  a  scientific  Chauvin.  Scientific 
Chauvinism,"  adds  the  learned  secretary,  "from  which 
German  investigators  have  hitherto  kept  free,  is  more 
reprehensible "  (gehassig)  than  political  Chauvinism,  inas- 
much as  self-control  {sittliche  Haltung)  is  more  to  be 
expected  from  men  of  science  than  from  the  politically 
excited  mass."  ' 

*  Festrede,  delivered  before  the  Academy  of  Sciences  of  Berlin,  in  celebra- 
tion of  the  birthday  of  the  Emperor  and  King,  March  28,  1878. 


406  FRAGMENTS   OF   SCIENCE 

In  the  case  before,  this  "expectation"  would,  I  fear, 
be  doomed  to  disappointment.  But  Du  Bois-Reymond 
and  his  countrymen  must  not  accept  the  writings  of  Pro- 
fessor Tait  as  representative  of  the  thought  of  England. 
Surely  no  nation  in  the  world  has  more  effectually  shaken 
itself  free  from  scientific  Chauvinism.  From  the  day  that 
Davy,  on  presenting  the  Copley  medal  to  Arago,  scorn- 
fully brushed  aside  that  spurious  patriotism  which  would 
run  national  boundaries  through  the  free  domain  of 
science,  chivalry  toward  foreigners  has  been  a  guiding 
principle   with   the   Royal    Society. 

On  the  more  private  amenities  indulged  in  by  Pro- 
fessor Tait,  I  do  not  consider  it  necessary  to  say  a  word. 


XVII 

CONTRIBUTIONS    TO    MOLECULAR    PHYSICS* 

HAYING  on  previous  occasions  dwelt  upon  the 
enormous  differences  which  exist  among  gaseous 
bodies  both  as  regards  their  power  of  absorbing 
and  emitting  radiant  heat,  I  have  now  to  consider  the 
effect  of  a  change  of  aggregation.  When  a  gas  is  con- 
densed to  a  liquid,  or  a  liquid  congealed  to  a  solid,  the 
molecules  coalesce  and  grapple  with  each  other  by  forces 
which  are  insensible  as  long  as  the  gaseous  state  is  main- 
tained. But,  even  in  the  solid  and  liquid  conditions,  the 
luminiferous  ether  still  surrounds  the  molecules:  hence, 
if  the  acts  of  radiation  and  absorption  depend  on  them 
individually,  regardless  of  their  state  of  aggregation,  the 
change  from  the  gaseous  to  the  liquid  state  ought  not  ma- 
terially to  affect  the  radiant  and  absorbent  power.  If,  on 
the  contrary,  the  mutual  entanglement  of  the  molecules 
by  the  force  of  cohesion  be  of  paramount  influence,  then 
we  may  expect  that  liquids  will  exhibit  a  deportment 
toward  radiant  heat  altogether  different  from  that  of  the 
vapors  from  which  they  are  derived. 

The  first  part  of  an  inquiry  conducted  in  1863-64  was 
devoted  to  an  exhaustive  examination  of  this  question. 
Twelve  different  liquids  were  employed,  and  ^ve  different 

'  A  discourse  delivered  at  the  Royal  Institution,  March  18,  1864 — suppl»- 
menthig,  though  of  prior  date,  the  Rede  Lecture  on  Radiation. 

(407) 


408  FRAGMENTS   OF  SCIENCE 

layers  of  each,  varying  in  thickness  from  0*02  of  an  inch 
to  0*27  of  an  inch.  The  liquids  were  enclosed,  not  in 
glass  vessels,  which  would  have  materially  modified  the 
incident  heat,  but  between  plates  of  transparent  rock-salt, 
which  only  slightly  affected  the  radiation.  The  source  of 
heat  throughout  these  comparative  experiments  consisted 
of  a  platinum  wire,  raised  to  incandescence  by  an  electric 
current  of  unvarying  strength.  The  quantities  of  radiant 
heat  absorbed  and  transmitted  by  each  of  the  liquids  at 
the  respective  thicknesses  were  first  determined.  The 
vapors  of  these  liquids  were  subsequently  examined,  the 
quantities  of  vapor  employed  being  rendered  proportional 
to  the  quantities  of  liquid  previously  traversed  by  the  ra- 
diant heat.  The  result  was  that,  for  heat  from  the  same 
source,  the  order  of  absorption  of  liquids  and  of  their 
vapors  proved  absolutely  the  same.  There  is  no  known 
exception  to  this  law;  so  that,  to  determine  the  position 
of  a  vapor  as  an  absorber  or  a  radiator,  it  is  only  nec- 
essary to  determine  the  position  of  its  liquid. 

This  result  proves  that  the  state  of  aggregation,  as  far, 
at  all  events,  as  the  liquid  stage  is  concerned,  is  of  alto- 
gether subordinate  moment — a  conclusion  which  will  prob- 
ably prove  to  be  of  cardinal  importance  in  molecular 
physics.  On  one  important  and  contested  point  it  has 
a  special  bearing.  If  the  position  of  a  liquid  as  an  ab- 
sorber and  radiator  determine  that  of  its  vapor,  the  posi- 
tion of  water  fixes  that  of  aqueous  vapor.  Water  has 
been  compared  with  other  liquids  in  a  multitude  of  ex- 
periments, and  it  has  been  found,  both  as  a  radiant  and 
as  an  absorbent,  to  transcend  them  all.  Thus,  for  exam- 
ple, a  layer  of  bisulphide  of  carbon  0*02  of  an  inch  in 
thickness  absorbs  6  per  cent  and  allows  94  per  cent  of 


CONTRIBUTIONS   TO   MOLECULAR   PHYSICS         409 

the  radiation  from  the  red-hot  platinum  spiral  to  pass 
through  it;  benzol  absorbs  43  and  transmits  57  per  cent 
of  the  same  radiation;  alcohol  absorbs  67  and  transmits 
88  per  cent,  and  alcohol,  as  an  absorber  of  radiant  heat, 
stands  at  the  head  of  all  liquids  except  one.  The  excep- 
tion is  water.  A  layer  of  this  substance,  of  the  thickness 
above  given,  absorbs  81  per  cent  and  permits  only  19  per 
cent  of  the  radiation  to  pass  through  it.  Had  no  single 
experiment  ever  been  made  upon  the  vapor  of  water,  its 
rigorous  action  upon  radiant  heat  might  be  inferred  from 
the  deportment  of  the  liquid. 

The  relation  of  absorption  and  radiation  to  the  chemi- 
cal constitution  of  the  radiating  and  absorbing  substances 
was  next  briefly  considered.  For  the  first  six  substances 
in  the  list  of  liquids  examined,  the  radiant  and  absorbent 
powers  augment  as  the  number  of  atoms  in  the  compound 
molecule  augments.  Thus,  bisulphide  of  carbon  has  3 
atoms,  chloroform  5,  iodide  of  ethyl  8,  benzol  12,  and 
amylene  15  atoms  in  their  respective  molecules.  The 
order  of  their  power  as  radiants  and  absorbents  is  that 
here  indicated,  bisulphide  of  carbon  being  the  feeblest 
and  amylene  the  strongest  of  the  six.  Alcohol,  however, 
excels  benzol  as  an  absorber,  though  it  has  but  9  atoms 
in  its  molecule;  but,  on  the  other  hand,  its  molecule  is 
rendered  more  complex  by  the  introduction  of  a  new  ele- 
ment. Benzol  contains  carbon  and  hydrogen,  while  alco- 
hol contains  carbon,  hydrogen  and  oxygen.  Thus,  not 
only  does  atomic  multitude  come  into  play  in  absorption 
and  radiation — atomic  complexity  must  also  be  taken  into 
account.  I  would  recommend  to  the  particular  attention 
of  chemists  the  molecule  of  water;  the  deportment  of  this 

substance  toward  radiant  heat  being  perfectly  anomalous, 

Science — Y— 18 


410  FRAGMENTS   OF  SCIENCE 

if  the  chemical  formula  at  present  ascribed  to  it  be 
correct. 

Sir  William  Herscbel  made  the  important  discovery 
that,  beyond  the  limits  of  the  red  end  of  the  solar  spec- 
trum, rays  of  high  heating  power  exist  which  are  incom- 
petent to  excite  vision.  The  discovery  is  capable  of  ex- 
tension. Dissolving  iodine  in  the  bisulphide  of  carbon,  a 
solution  is  obtained  which  entirely  intercepts  the  light  of 
the  most  brilliant  flames,  while  to  the  ultra- red  rays  of 
such  flames  the  same  iodine  is  found  to  be  perfectly  dia- 
thermic. The  transparent  bisulphide,  which  is  highly 
pervious  to  invisible  heat,  exercises  on  it  the  same  ab- 
sorption as  the  perfectly  opaque  solution.  A  hollow  prism 
filled  with  the  opaque  liquid  being  placed  in  the  path  of 
the  beam  from  an  electric  lamp,  the  light-spectrum  is  com- 
pletely intercepted,  but  the  heat- spectrum  may  be  received 
upon  a  screen  and  there  examined.  Falling  upon  a 
thermo-electric  pile,  its  invisible  presence  is  shown  by 
the  prompt  deflection  of  even  a  coarse  galvanometer. 

What,  then,  is  the  physical  meaning  of  opacity  and 
transparency  as  regards  light  and  radiant  heat?  The  vis- 
ible ray^  of  the  spectrum  differ  from  the  invisible  ones 
simply  in  period.  The  sensation  of  light  is  excited  by 
waves  of  ether  shorter  and  more  quickly  recurrent  than 
the  non-visual  waves  which  fall  beyond  the  extreme  red. 
But  why  should  iodine  stop  the  former  and  allow  the 
latter  to  pass?  The  answer  to  this  question  no  doubt 
is,  that  the  intercepted  waves  are  those  whose  periods  of 
recurrence  coincide  with  the  periods  of  oscillation  possible 
to  the  atoms  of  the  dissolved  iodine.  The  elastic  forces 
which  keep  these  atoms  apart  compel  them  to  vibrate  in 
definite  periods,  and,  when  these  periods  synchronize  with 


CONTRIBUTIONS   TO   MOLECULAR   PHYSICS         411 

those  of  the  ethereal  waves,  the  latter  are  absorbed. 
Briefly  defined,  then,  transparency  in  liquids,  as  well 
as  in  gases,  is  synonjnnous  with  discord,  while  opacity 
is  synonymous  with  accord,  between  the  periods  of  the 
waves  of  ether  and  those  of  the  molecules  on  which  they 
impinge. 

According  to  this  view  transparent  and  colorless  sub- 
stances  owe  their  transparency  to  the  dissonance  existing 
between  the  oscillating  periods  of  their  atoms  and  those 
of  the  waves  of  the  whole  visible  spectrum.  From  the 
prevalence  of  transparency  in  compound  bodies,  the  gen- 
eral discord  of  the  vibrating  periods  of  their  atoms  with 
the  light-giving  waves  of  the  spectrum  may  be  inferred; 
while  their  synchronism  with  the  ultra-red  periods  is  to  be 
inferred  from  their  opacity  to  the  ultra-red  rays.  Water 
illustrates  this  in  a  most  striking  manner.  It  is  highly 
transparent  to  the  luminous  rays,  which  proves  that  its 
atoms  do  not  readily  oscillate  in  the  periods  which  excite 
vision.  It  is  highly  opaque  to  the  ultra-red  undulations, 
which  proves  the  synchronism  of  its  vibrating  periods 
with  those  of  th^  longer  waves. 

If,  then,  to  the  radiation  from  any  source  water  shows 
itself  eminently  or  perfectly  opaque,  we  may  infer  that  the 
atoms  whence  the  radiation  emanates  oscillate  in  ultra-red 
periods.  Let  us  apply  this  test  to  the  radiation  from  a 
flame  of  hydrogen.  This  flame  consists  mainly  of  incan- 
descent aqueous  vapor,  the  temperature  of  which,  as  cal- 
culated by  Bunsen,  is  3,269°  C,  so  that,  if  the  penetrative 
power  of  radiant  heat,  as  generally  supposed,  augment 
with  the  temperature  of  its  source,  we  may  expect  the 
radiation  from  this  flame  to  be  copiously  transmitted  by 
water.     While,  however,  a  layer  of  the  bisulphide  of  car- 


412  FRAGMENTS   OF  SCIENCE 

bon  0*07  of  an  inch  in  thickness  transmits  72  per  cent  of 
the  incident  radiation,  and  while  every  other  liquid  exam- 
ined transmits  more  or  less  of  the  heat,  a  layer  of  water 
of  the  above  thickness  is  entirely  opaque  to  the  radiation 
from  the  hydrogen  flame.  Thus  we  establish  accord  be- 
tween the  periods  of  the  atoms  of  cold  water  and  those 
of  aqueous  vapor  at  a  temperature  of  8,259°  C.  But  the 
periods  of  water  have  already  been  proved  to  be  ultra- 
red — hence  those  of  the  hydrogen  flame  must  be  sensibly 
ultra-red  also.  The  absorption  by  dry  air  of  the  heat 
emitted  by  a  platinum  spiral  raised  to  incandescence  by 
electricity  is  insensible,  while  that  by  the  ordinary  undried 
air  is  6  per  cent.  Substituting  for  the  platinum  spiral  a 
hydrogen  flame,  the  absorption  by  dry  air  still  remains 
insensible,  while  that  of  the  undried  air  rises  to  20  per 
cent  of  the  entire  radiation.  The  temperature  of  the  hy- 
drogen flame  is,  as  stated,  3,259°  C. ;  that  of  the  aqueous 
vapor  of  the  air  20°  C.  Suppose,  then,  the  temperature  of 
aqueous  vapor  to  rise  from  20°  C.  to  8,259°  C,  we  must 
conclude  that  the  augmentation  of  temperature  is  applied 
to  an  increase  of  amplitude  or  width  of  swing,  and  not  to 
the  introduction  of  quicker  periods  into  the  radiation. 

The  part  played  by  aqueous  vapor  in  the  economy  of 
nature  is  far  more  wonderful  than  has  been  hitherto  sup- 
posed. To  nourish  the  vegetation  of  the  earth  the  actinic 
and  luminous  rays  of  the  sun  must  penetrate  our  atmos- 
phere; and  to  such  rays  aqueous  vapor  is  eminently  trans- 
parent. The  violet  and  the  ultra-violet  rays  pass  through 
it  with  freedom.  To  protect  vegetation  from  destructive 
chills  the  terrestrial  rays  must  be  checked  in  their  transit 
toward  stellar  space;  and  this  is  accomplished  by  the 
aqueous   vapor  di£Eused   through  the  air.     This  substance 


CONTRIBUTIONS   TO   MOLECULAR   PHYSICS         413 

is  the  great  moderator  of  the  earth's  temperature,  bring- 
ing its  extremes  into  proximity,  and  obviating  contrasts 
between  day  and  night  which  would  render  life  insupport- 
able. But  we  can  advance  beyond  this  general  statement, 
now  that  we  know  the  radiation  from  aqueous  vapor  is 
intercepted,  in  a  special  degree,  by  water,  and,  recipro- 
cally, the  radiation  from  water  by  aqueous  vapor;  for  it 
follows  from  this  that  the  very  act  of  nocturnal  refrigera- 
tion which  produces  the  condensation  of  aqueous  vapor  at 
the  surface  of  the  earth — giving,  as  it  were,  a  varnish  of 
water  to  that  surface — imparts  to  terrestrial  radiation  that 
particular  character  which  disqualifies  it  from  passing 
through  the  earth's  atmosphere  and  losing  itself  in  space. 
And  here  we  come  to  a  question  in  molecular  physics 
which  at  the  present  moment  occupies  attention.  By  al- 
lowing the  violet  and  ultra-violet  rays  of  the  spectrum  to 
fall  upon  sulphate  of  quinine  and  other  substances.  Pro- 
fessor Stokes  has  changed  the  periods  of  those  rays.  At- 
tempts have  been  made  to  produce  a  similar  result  at  the 
other  end  of  the  spectrum — to  convert  the  ultra -red  periods 
into  periods  competent  to  excite  vision — but  hitherto  with- 
out success.  Such  a  change  of  period,  I  agree  with  Dr. 
Miller  in  believing,  occurs  when  the  lime-light  is  produced 
by  an  oxy-hydrogen  flame.  In  this  common  experiment 
there  is  an  actual  breaking  up  of  long  periods  into  short 
ones — a  true  rendering  of  un visual  periods  visual.  The 
change  of  refrangibility  here  effected  differs  from  that  of 
Professor  Stokes;  first,  by  its  being  in  the  opposite  di- 
rection— that  is,  from  a  lower  refrangibility  to  a  higher; 
and,  secondly,  in  the  circumstance  that  the  lime  is  heated 
by  the  collision  of  the  molecules  of  aqueous  vapor,  before 
their  heat  has  assumed  the  radiant  form.     But  it  cannot 


414  FRAGMENTS   OF  SCIENCE 

be  doubted  that  the  same  effect  would  be  produced  by 
radiant  beat  of  the  same  periods,  provided  the  motion 
of  the  ether  could  be  rendered  sufficiently  intense.*  The 
effect  in  principle  is  the  same,  whether  we  consider  the 
lime  to  be  struck  by  a  particle  of  aqueous  vapor  oscillat- 
ing at  a  certain  rate,  or  by  a  particle  of  ether  oscillating 
at  the  same  rate. 

By  plunging  a  platinum  wire  into  a  hydrogen  flame  we 
cause  it  to  glow,  and  thus  introduce  shorter  periods  into 
the  radiation.  These,  as  already  stated,  are  in  discord 
with  the  atomic  vibrations  of  water;  hence  we  may  infer 
that  the  transmission  through  water  will  be  rendered  more 
copious  by  the  introduction  of  the  wire  into  the  flame. 
Experiment  proves  this  conclusion  to  be  true.  Water, 
from  being  opaque,  opens  a  passage  to  6  per  cent  of  the 
radiation  from  the  spiral.  A  thin  plate  of  colorless  glass, 
moreover,  transmits  68  per  cent  of  the  radiation  from  the 
hydrogen  flame;  but  when  the  flame  and  spiral  are  em- 
ployed, 78  per  cent  of  the  heat  is  transmitted. 

For  an  alcohol  flame  Knoblauch  and  Melloni  found 
glass  to  be  less  transparent  than  for  the  same  flame  with 
a  platinum  spiral  immersed  in  it;  but  Melloni  afterward 
showed  that  the  result  was  not  general^ — that  black  glass 
and  black  mica  were  decidedly  more  diathermic  to  the 
radiation  from  the  pure  alcohol  flame.  Melloni  did  not 
explain  this,  but  the  reason  is  now  obvious.  The  mica 
and  glass  owe  their  blackness  to  the  carbon  diffused 
through  them.  This  carbon,  as  first  proved  by  Melloni, 
is  in  some  measure  transparent  to  the  ultra-red  rays,  and 
I  have  myself    succeeded  in  transmitting  between  40  and 

*  This  was  soon  afterwaxd  accomplished.     See  pp.  53-55. 


CONTRIBUTIONS   TO   MOLECULAR   PHYSICS         415 

50  per  cent  of  the  radiation  from  a  hydrogen  flame 
through  a  layer  of  carbon  which  intercepted  the  light  of 
an  intensely  brilliant  flame.  The  products  of  combustion 
of  alcohol  are  carbonic  acid  and  aqueous  vapor,  the  heat 
of  which  is  almost  wholly  ultra-red.  For  this  radiation, 
then,  the  carbon  is  in  a  considerable  degree  transparent, 
while  for  the  radiation  from  the  platinum  spiral  it  is  in 
a  great  measure  opaque.  The  platinum  wire,  therefore, 
which  augmented  the  radiation  through  the  pure  glass, 
augmented  the  absorption  of  the  black  glass  and  mica. 

No  more  striking  or  instructive  illustration  of  the  in- 
fluence of  coincidence  could  be  adduced  than  that  fur- 
nished by  the  radiation  from  a  carbonic  oxide  flame. 
Here  the  product  of  combustion  is  carbonic  acid;  and  on 
the  radiation  from  this  flame  even  the  ordinary  carbonic 
acid  of  the  atmosphere  exerts  a  powerful  effect.  A  quan- 
tity of  the  gas,  only  one-thirtieth  of  an  atmosphere  in 
density,  contained  in  a  polished  brass  tube  four  feet  long, 
intercepts  60  per  cent  of  the  radiation  from  the  carbonic 
oxide  flame.  For  the  heat  emitted  by  lampblack,  olefiant 
gas  is  a  far  more  powerful  absorber  than  carbonic  acid; 
in  fact,  for  such  heat,  with  one  exception,  carbonic  acid 
is  the  most  feeble  absorber  to  be  found  among  the  com- 
pound gases.  Moreover,  for  the  radiation  from  a  hydro- 
gen flame  olefiant  gas  possesses  twice  the  absorbent  power 
of  carbonic  acid,  while  for  the  radiation  from  the  carbonic 
oxide  flame,  at  a  common  pressure  of  one  inch  of  mercury, 
the  absorption  by  carbonic  acid  is  more  than  twice  that 
of  olefiant  gas.  Thus  we  establish  the  coincidence  of 
period  between  carbonic  acid  at  a  temperature  of  20°  C. 
and  carbonic  acid  at  a  temperature  of  over  3,000°  C, 
the   periods  of   oscillation  of    both  the   incandescent  and 


416  FRAGMENTS   OF  SCIENCE 

the  cold   gas   belonging  to  the   ultra-red   portion   of   the 
spectrum. 

It  will  be  seen  from  the  foregoing  remarks  and  experi- 
ments how  impossible  it  is  to  determine  the  effect  of  tem- 
perature pure  and  simple  on  the  transmission  of  radiant 
heat  if  different  sources  of  heat  be  employed.  Through- 
out such  an  examination  the  same  oscillating  atoms  ought 
to  be  retained.  This  is  done  by  heating  a  platinum  spiral 
by  an  electric  current,  the  temperature  meanwhile  varying 
between  the  widest  possible  limits.  Their  comparative 
opacity  to  the  ultra-red  rays  shows  the  general  accord  of 
the  oscillating  periods  of  the  vapors  referred  to  at  the  com- 
mencement of  this  lecture  with  those  of  the  ultra-red  un- 
dulations. Hence,  by  gradually  heating  a  platinum  wire 
from  darkness  up  to  whiteness,  we  ought  gradually  to 
augment  the  discord  between  it  and  these  vapors,  and 
thus  augment  the  transmission.  Experiment  entirely  con- 
firms this  conclusion.  Formic  ether,  for  example,  absorbs 
45  per  cent  of  the  radiation  from  a  platinum  spiral  heated 
to  barely  visible  redness;  82  per  cent  oi  the  radiation 
from  the  same  spiral  at  a  red  heat;  26  per  cent  of  the 
radiation  from  a  white-hot  spiral,  and  only  21  per  cent 
when  the  spiral  is  brought  near  its  point  of  fusion.  Ke- 
markable  cases  of  inversion  as  to  transparency  also  occur. 
For  barely  visible  redness  formic  ether  is  more  opaque 
than  sulphuric;  for  a  bright  red  heat  both  are  equalljp 
transparent;  while,  for  a  white  heat,  and  still  more  for  a 
higher  temperature,  sulphuric  ether  is  more  opaque  than 
formic.  This  result  gives  us  a  clear  view  of  the  relation^ 
ship  of  the  two  substances  to  the  luminiferous  ether.  As 
we  introduce  waves  of  shorter  period  the  sulphuric  ethet 
augments  most  rapidly  in  opacity;  that  is  to  say,  its  accord 


CONTRIBUTIONS    TO   MOLECULAR   PHYSICS         417 

with  the  shorter  waves  is  greater  than  that  of  the  formic. 
Hence  we  may  infer  that  the  atoms  of  formic  ether  oscil- 
late, on  the  whole,  more  slowly  than  those  of  sulphuric 
ether. 

When  the  source  of  heat  is  a  Leslie's  cube  coated  with 
lampblack  and  filled  with  boiling  water,  the  opacity  of 
formic  ether  in  comparison  with  sulphuric  is  very  de- 
cided. With  this  source  also  the  positions  of  chloroform 
and  iodide  of  methyl  are  inverted.  For  a  white-hot  spiral, 
the  absorption  of  chloroform  vapor  being  10  per  cent, 
that  of  iodide  of  methyl  is  16;  with  the  blackened  cube 
as  source,  the  absorption  by  chloroform  is  22  per  cent, 
while  that  by  the  iodide  of  methyl  is  only  19.  This  in- 
version is  not  the  result  of  temperature  merely;  for  when 
a  platinum  wire,  heated  to  the  temperature  of  boiling 
water,  is  employed  as  a  source,  the  iodide  continues  to 
be  the  most  powerful  absorber.  All  the  experiments 
hitherto  made  go  to  prove  that  from  heated  lampblack  an 
emission  takes  place  which  synchronizes  in  an  especial 
manner  with  chloroform.  For  the  cube  at  100°  C,  coated 
with  lampblack,  the  absorption  by  chloroform  is  more 
than  three  times  that  by  bisulphide  of  carbon;  for  the 
radiation  from  the  most  luminous  portion  of  a  gas-flame 
the  absorption  by  chloroform  is  also  considerably  in  ex- 
cess of  that  by  bisulphide  of  carbon;  while,  for  the  flame 
of  a  Bunsen's  burner,  from  which  the  incandescent  carbon 
particles  are  removed  by  the  free  admixture  of  air,  the 
absorption  by  bisulphide  of  carbon  is  nearly  twice  that  by 
chloroform.  The  removal  of  the  carbon  particles  more  than 
doubles  the  relative  transparency  of  the  chloroform.  Test- 
ing, moreover,  the  radiation  from  various  parts  of  the 
Bame  flame,   it   was  found  that  for  the  blue  base  of  the 


418  FRAGMENTS   OF  SCIENCE 

flame  the  bisulphide  of  carbon  was  most  opaque,  while 
for  all  other  parts  of  the  flame  the  chloroform  was  most 
opaque.  For  the  radiation  from  a  very  small  gas- flame, 
consisting  of  a  blue  base  and  a  small  white  tip,  the  bisul- 
phide was  also  most  opaque,  and  its  opacity  very  decid- 
edly exceeded  that  of  the  chloroform  when  the  source  of 
heat  was  the  flame  of  bisulphide  of  carbon.  Comparing 
the  radiation  from  a  Leslie's  cube  coated  with  isinglass 
with  that  from  a  similar  cube  coated  with  lampblack,  at 
the  common  temperature  of  100°  C,  it  was  found  that, 
out  of  eleven  vapors,  all  but  one  absorbed  the  radiation 
from  the  isinglass  most  powerfully;  the  single  exception 
was  chloroform. 

It  is  worthy  of  remark  that  whenever,  through  a  change 
of  source,  the  position  of  a  vapor  as  an  absorber  of  radiant 
heat  was  altered,  the  position  of  the  liquid  from  which  the 
vapor  was  derived  underwent  a  similar  change. 

It  is  still  a  point  of  difference  between  eminent  investi- 
gators whether  radiant  heat,  up  to  a  temperature  of  100° 
C,  is  monochromatic  or  not.  Some  afiirm  this;  some 
deny  it.  A  long  series  of  experiments  enables  me  to  state 
that  probably  no  two  substances  at  a  temperature  of  100* 
0.  emit  heat  of  the  same  quality.  The  heat  emitted  by 
isinglass,  for  example,  is  different  from  that  emitted  by 
lampblack,  and  the  heat  emitted  by  cloth,  or  paper,  differs 
from  both.  It  is  also  a  subject  of  discussion  whether  rock- 
salt  is  equally  diathermic  to  all  kinds  of  calorific  rays;  the 
differences  affirmed  to  exist  by  some  investigators  being 
ascribed  by  others  to  differences  of  incidence  from  the 
various  sources  employed.  MM.  de  la  Provostaye  and 
Deeains  maintain  the  former  view,  Melloni  and  M.  Knob- 
lauch maintain  the  latter.      I   tested   this   point  without 


CONTRIBUTIONS   TO   MOLECULAR   PHYSICS         419 

changing  anything  but  the  temperature  of  the  source;  its 
size,  distance,  and  surroundings  remaining  the  same.  The 
experiments  proved  rock-salt  to  be  colored  thermally.  It 
is  more  opaque,  for  example,  to  the  radiation  from  a  barely 
visible  spiral  than  to  that  from  a  white-hot  one. 

In  regard,  to  the  relation  of  radiation  to  conduction,  if 
we  define  radiation,  internal  as  well  as  external,  as  the 
communication  of  motion  from  the  vibrating  atoms  to  the 
ether,  we  may,  I  think,  by  fair  theoretic  reasoning,  reach 
the  conclusion  that  the  best  radiators  ought  to  prove  the 
worst  conductors.  A  broad  consideration  of  the  subject 
shows  at  once  the  general  harmony  of  this  conclusion  with 
observed  facts.  Organic  substances  are  all  excellent  radia- 
tors; they  are  also  extremely  bad  conductors.  The  mo- 
ment we  pass  from  the  metals  to  their  compounds  we  pass 
from  good  conductors  to  bad  ones,  and  from  bad  radiators 
to  good  ones.  Water,  among  liquids,  is  probably  the 
worst  conductor;  it  is  the  best  radiator.  Silver,  among 
solids,  is  the  best  conductor;  it  is  the  worst  radiator.  The 
excellent  researches  of  MM.  de  la  Provostaye  and  Desains 
furnish  a  striking  illustration  of  what  I  am  inclined  to 
regard  as  a  natural  law — that  those  atoms  which  transfer 
the  greatest  amount  of  motion  to  the  ether,  or,  in  other 
words,  radiate  most  powerfully,  are  the  least  competent 
to  communicate  motion  to  each  other,  or,  in  other  words. 
to  propagate  by  conduction  readily. 


XYIII 

LIFE    AND    LETTERS    OF    FARADAY 
18'70 

UNDERTAKEN  and  executed  in  a  reverent  and  lov- 
ing spirit,  tlie  work  of  Dr.  Bence  Jones  makes 
Faraday  the  virtual  writer  of  his  own  life.  Every- 
body now  knows  the  story  of  the  philosopher's  birth; 
that  his  father  was  a  smith;  that  he  was  bom  at  Kew- 
ington  Butts  in  1791 ;  that  he  ran  along  the  London  pave- 
ments, a  bright- eyed  errand  boy,  with  a  load  of  brown 
curls  upon  his  head  and  a  packet  of  newspapers  under 
his  arm;  that  the  lad's  master  was  a  bookseller  and  book- 
binder— a  kindly  man,  who  became  attached  to  the  little 
fellow,  and  in  due  time  made  him  his  apprentice  without 
fee;  that  during  his  apprenticeship  he  found  his  appetite 
for  knowledge  provoked  and  strengthened  by  the  books 
he  stitched  and  covered.  Thus  he  grew  in  wisdom  and 
stature  to  his  year  of  legal  manhood,  when  he  appears  in 
the  volumes  before  us  as  a  writer  of  letters,  which  reveal 
his  occupation,  acquirements,  and  tone  of  mind.  His 
correspondent  was  Mr.  Abbott,  a  member  of  the  Society 
of  Friends,  who,  with  a  forecast  of  his  correspondent's 
greatness,  preserved  his  letters  and  produced  them  at  the 
proper  time. 

In  later  years  Faraday  always  carried  in  his  pocket  a 
blank  card,  on  which  he  jotted  down  in  pencil  his  thoughts 
(420) 


LIFE   AND   LETTERS   OF  FARADAY  421 

and  memoranda.  He  made  his  notes  in  the  laboratory,  in 
the  theatre,  and  in  the  streets.  This  distrust  of  his  mem- 
ory reveals  itself  in  his  first  letter  to  Abbott.  To  a  propo- 
sition that  no  new  inquiry  should  be  started  between  them 
before  the  old  one  had  been  exhaustively  discussed,  Fara- 
day objects.  "Your  notion,"  he  says,  "I  can  hardly  al- 
low, for  the  following  reason:  ideas  and  thoughts  spring 
up  in  my  mind  which  are  irrevocably  lost  for  want  of 
noting  at  the  time."  Grentle  as  he  seemed,  he  wished 
to  have  his  own  way,  and  he  had  it  throughout  his  life. 
Differences  of  opinion  sometimes  arose  between  the  two 
friends,  and  then  they  resolutely  faced  each  other.  '*I 
accept  your  offer  to  fight  it  out  with  joy,  and  shall  in  the 
battle  of  experience  cause  not  pain,  but,  I  hope,  pleas- 
ure." Faraday  notes  his  own  impetuosity,  and  inces- 
santly checks  it.  There  is  at  times  something  almost 
mechanical  in  his  self-restraint.  In  another  nature  it 
would  have  hardened  into  mere  "correctness"  of  conduct; 
but  his  overflowing  affections  prevented  this  in  his  case. 
The  habit  of  self-control  became  a  second  nature  to  him 
at  last,  and  lent  serenity  to  his  later  years. 

In  October,  1812,  he  was  engaged  by  a  Mr.  De  la  Roche 
as  a  journeyman  bookbinder;  but  the  situation  did  not 
suit  him.  His  master  appears  to  have  been  an  austere 
and  passionate  man,  and  Faraday  was  to  the  last  degree 
sensitive.  All  his  life  he  continued  so.  He  suffered  at 
times  from  dejection;  and  a  certain  grimness,  too,  per- 
vaded his  moods.  "At  present,"  he  writes  to  Abbott,  "I 
am  as  serious  as  you  can  be,  and  would  not  scruple  to 
speak  a  truth  to  any  human  being,  whatever  repugnance 
it  might  give  rise  to.  Being  in  this  state  of  mind,  I 
should   have   refrained   from    writing   to   you,   did   I   not 


422  FRAGMENTS   OF  SCIENCE 

conceive  from  the  general  tenor  of  your  letters  that  your 
mind  is,  at  proper  times,  occupied  upon  serious  subjects 
to  the  exclusion  of  those  that  are  frivolous."  Plainly 
he  had  fallen  into  that  stern  Puritan  mood,  which  not 
only  crucifies  the  affections  and  lusts  of  him  who  har- 
bors it,  but  is  often  a  cause  of  disturbed  digestion  to 
his   friends. 

About  three  months  after  his  engagement  with  De  la 
Koche,  Faraday  quitted  him  and  bookbinding  together. 
He  had  heard  Davy,  copied  his  lectures,  and  written  to 
him,  entreating  to  be  released  from  Trade,  which  he 
hated,  and  enabled  to  pursue  Science.  Davy  recognized 
the  merit  of  his  correspondent,  kept  his  eye  upon  him, 
and,  when  occasion  offered,  drove  to  his  door  and  sent  in 
a  letter,  offering  him  the  post  of  assistant  in  the  laboratory 
of  the  Eoyal  Institution.  He  was  engaged  March  1,  1813, 
and  on  the  8th  we  find  him  extracting  the  sugar  from 
beet-root.  He  joined  the  City  Philosophical  Society  which 
had  been  founded  by  Mr.  Tatum  in  1808.  "The  discipline 
was  very  sturdy,  the  remarks  very  plain,  and  the  results 
most  valuable."  Faraday  derived  great  profit  from  this 
little  association.  In  the  laboratory  he  had  a  discipline 
sturdier  still.  Both  Davy  and  himself  were  at  this  time 
frequently  cut  and  bruised  by  explosions  of  chloride  of 
nitrogen.  One  explosion  was  so  rapid  "as  to  blow  my 
hand  open,  tear  away  a  part  of  one  nail,  and  make  my 
fingers  so  sore  that  I  cannot  use  them  easily."  In  an- 
other experiment  "the  tube  and  receiver  were  blown  to 
pieces,  I  got  a  cut  on  the  head,  and  Sir  Humphry  a 
bruise  on  his  hand."  And  again  speaking  of  the  same 
substance,  he  says,  "when  put  in  the  pump  and  ex- 
hausted, it  stood  for  a  moment,   and  then  exploded  with 


LIFE   AND    LETTERS    OF   FARADAY  428 

a  fearful  noise.  Both  Sir  H.  and  I  had  masks  on,  but  I 
escaped  this  time  the  best.  Sir  H.  had  his  face  cut  in 
two  places  about  the  chin,  and  a  violent  blow  on  the 
forehead  struck  through  a  considerable  thickness  of  silk 
and  leather."  It  was  this  same  substance  that  blew  out 
the  eye  of  Dulong. 

Over  and  over  again,  even  at  this  early  date,  we  can 
discern  the  quality  which,  compounded  with  his  rare  in- 
tellectual power,  made  Faraday  a  great  experimental  phi- 
losopher. This  was  his  desire  to  see  facts,  and  not  to  rest 
contented  with  the  descriptions  of  them.  He  frequently 
pits  the  eye  against  the  ear,  and  affirms  the  enormous  su- 
periority of  the  organ  of  vision.  Late  in  life  I  have  heard 
him  say  that  he  could  never  fully  understand  an  experi- 
ment until  he  had  seen  it.  But  he  did  not  confine  himself 
to  experiment.  He  aspired  to  be  a  teacher,  and  reflected 
and  wrote  upon  the  method  of  scientific  exposition.  '*A 
lecturer,"  he  observes,  ** should  appear  easy  and  collected, 
undaunted  and  unconcerned:"  still  '*his  whole  behavior 
should  evince  respect  for  his  audience."  These  recom- 
mendations were  afterward,  in  great  part,  embodied  by 
himself.  I  doubt  his  ** unconcern,"  but  his  fearlessness 
was  often  manifested.  It  used  to  rise  within  him  as  a 
wave,  which  carried  both  him  and  his  audience  along 
with  it.  On  rare  occasions  also,  when  he  felt  himself  and 
his  subject  hopelessly  unintelligible,  he  suddenly  evoked 
a  certain  recklessness  of  thought,  and,  without  halting  to 
extricate  his  bewildered  followers,  he  would  dash  alone 
through  the  jungle  into  which  he  had  unwittingly  led 
them;  thus  saving  them  from  ennui  by  the  exhibition  of 
a  vigor  which,  for  the  time  being,  they  could  neither  share 
nor  comprehend. 


424  FRAGMENTS   OF  SCIENCE 

In  October,  1818,  he  quitted  England  with  Sir  Humphry 
and  Lady  Davy.  During  his  absence  he  kept  a  journal, 
from  which  copious  and  interesting  extracts  have  been 
made  by  Dr.  Bence  Jones.  Davy  was  considerate,  pre- 
ferring at  times  to  be  his  own  servant  rather  than  impose 
on  Faraday  duties  which  he  disliked.  But  Lady  Davy 
was  the  reverse.  She  treated  him  as  an  underling;  he 
chafed  under  the  treatment,  and  was  often  on  the  point 
of  returning  home.  They  halted  at  Q-eneva.  De  la  Eive, 
the  elder,  had  known  Davy  in  1799,  and,  by  his  writings 
in  the  "  Biblioth^que  Britannique, "  had  been  the  first  to 
make  the  English  chemist's  labors  known  abroad.  He 
welcomed  Davy  to  his  country  residence  in  1814.  Both 
were  sportsmen,  and  they  often  went  out  shooting  to- 
gether. 

On  these  occasions  Faraday  charged  Davy's  gun  while 
De  la  Rive  charged  his  own.  Once  the  G-enevese  phi- 
losopher found  himself  by  the  side  of  Faraday,  and,  in 
his  frank  and  genial  way,  entered  into  conversation  with 
the  young  man.  It  was  evident  that  a  person  possessing 
such  a  charm  of  manner  and  such  high  intelligence  could 
be  no  mere  servant.  On  inquiry  De  la  Rive  was  some- 
what shocked  to  find  that  the  soi-disant  domestique  was 
really  preparateur  in  the  laboratory  of  the  Royal  Institu- 
tion; and  he  immediately  proposed  that  Faraday  thence- 
forth should  join  the  masters  instead  of  the  servants  at 
their  meals.  To  this  Davy,  probably  out  of  weak  defer- 
ence to  his  wife,  objected;  but  an  arrangement  was  come 
to  that  Faraday  thenceforward  should  have  his  food  in 
his  own  room.  Rumor  states  that  a  dinner  in  honor  of 
Faraday  was  given  by  De  la  Rive.  This  is  a  delusion; 
there  was  no  such  banquet;  but  Faraday  never  forgot  the 


LIFE  AND   LETTERS   OF  FARADAY  425 

idndness  of  the  friend  who  saw  his  merit  when  he  was  a 
mere  gargon  de  lahoratoire.^ 

He  returned,  in  1815,  to  the  Rojal  Institution.  Here 
he  helped  Davy  for  years;  he  worked  also  for  himself, 
and  lectured  frequently  at  the  City  Philosophical  Society. 
He  took  lessons  in  elocution,  happily  without  damage  to 
his  natural  force,  earnestness,  and  grace  of  delivery.  He 
was  never  pledged  to  theory,  and  he  changed  in  opinion 
as  knowledge  advanced.  With  him  life  was  growth.  In 
those  early  lectures  we  hear  him  say,  **In  knowledge,  that 
man  only  is  to  be  contemned  and  despised  who  is  not  in 
a  state  of  transition.'*  And  again:  "Nothing  is  more  dif- 
ficult and  requires  more  caution  than  philosophical  deduc»- 
tion,  nor  is  there  anything  more  adverse  to  its  accuracy 
than  fixity  of  opinion."  Not  that  he  was  wafted  about 
by  every  wind  of  doctrine;  but  that  he  united  flexibility 
with  his  strength.  In  striking  contrast  with  this  intel- 
lectual expansiveness  was  bis  fixity  in  religion,  but  this 
is  a  subject  which  cannot  be  discussed  here. 

Of  all  the  letters  published  in  these  volumes  none  pos- 
sess a  greater  charm  than  those  of  Faraday  to  his  wife. 
Here,  as  Dr.  Bence  Jones  truly  remarks,  *'he  laid  open  all 
his  mind  and  the  whole  of  his  character,  and  what  can  be 
made  known  can  scarcely  fail  to  charm  every  one  by  ita 
loveliness,  its  truthfulness,  and  its  earnestness."     Abbott 


*  While  confined  last  autumn  at  Geneva  by  the  efEects  of  a  fall  in  the  Alps, 
my  friends,  with  a  kindness  I  can  never  forget,  did  all  that  friendship  could 
suggest  to  render  my  captivity  pleasant  to  me.  M.  de  la  Rive  then  wrote  out 
for  me  the  full  account,  of  which  the  foregoing  is  a  condensed  abstract.  Il 
was  at  the  desire  of  Dr.  Bence  Jones  that  I  asked  him  to  do  so.  The  rum* 
of  a  banquet  at  Gteneva  illustrates  the  tendency  to  substitute  for  the  youth  dl 
18X4  tiis  Faraday  of  later  years. 


426  FRAGMENTS   OF  SCIENCE 

and  he  sometimes  swerved  into  word-play  about  love; 
but  up  to  1820,  or  thereabout,  the  passion  was  potential 
merely.  Faraday's  journal,  indeed,  contains  entries  which 
show  that  he  took  pleasure  in  the  assertion  of  his  con- 
tempt for  love;  but  these  very  entries  became  links  in  his 
destiny.  It  was  through  them  that  he  became  acquainted 
with  one  who  inspired  him  with  a  feeling  which  only 
ended  with  his  life.  His  biographer  has  given  us  the 
means  of  tracing  the  varying  moods  which  preceded  his 
acceptance.  They  reveal  more  than  the  common  alterna- 
tions of  light  and  gloom;  at  one  moment  he  wishes  that 
his  flesh  might  melt  and  that  he  might  become  nothing; 
at  another  he  is  intoxicated  with  hope.  The  impetuosity 
of  his  character  was  then  unchastened  by  the  discipline  to 
which  it  was  subjected  in  after  years.  The  very  strength 
of  his  passion  proved  for  a  time  a  bar  to  its  advance, 
suggesting,  as  it  did,  to  the  conscientious  mind  of  Miss 
Barnard,  doubts  of  her  capability  to  return  it  with  ade- 
quate force.  But  they  met  again  and  again,  and  at  each 
successive  meeting  he  found  his  heaven  clearer,  until  at 
length  he  was  able  to  say,  "Not  a  moment's  alloy  of  this 
evening's  happiness  occurred.  Everything  was  delightful 
to  the  last  moment  of  my  stay  with  my  companion,  be- 
cause she  was  so.**  The  turbulence  of  doubt  subsided, 
and  a  calm  and  elevating  confidence  took  its  place. 
•*What  can  1  call  myself,"  he  writes  to  her  in  a  sub- 
sequent letter,  "to  convey  most  perfectly  my  affection  and 
love  for  yoil?  Can  I  or  can  truth  say  more  than  that  for 
this  world  I  am  yours?"  Assuredly  he  made  his  pro- 
fession good,  and  no  fairer  light  falls  upon  his  character 
than  that  which  reveals  his  relations  to  his  wife.  Never, 
1  believe,  existed  a  manlier,  purer,  steadier  love.     Like  a 


LIFE   AND   LETTERS    OF  FARADAY  427 

burning  diamond,   it  continued  to  shed,  for  six-and-forty 
years,  its  white  and  smokeless  glow. 

Faraday  was  married  on  June  12,  1821;  and  up  to  this 
date  Davy  appears  throughout  as  his  friend.  Soon  after- 
ward, however,  disunion  occurred  between  them,  which, 
while  it  lasted,  must  have  given  Faraday  intense  pain.  It 
is  impossible  to  doubt  the  honesty  of  conviction  with 
which  this  subject  has  been  treated  by  Dr.  Bence  Jones, 
and  there  may  be  facts  known  to  him,  but  not  appearing 
in  these  volumes,  which  justify  his  opinion  that  Davy  in 
those  days  had  become  jealous  of  Faraday.  This,  which 
is  the  prevalent  belief,  is  also  reproduced  in  an  excellent 
article  in  the  March  number  of  "Fraser's  Magazine." 
But  the  best  analysis  I  can  make  of  the  data  fails  to 
present  Davy  in  this  light  to  me.  The  facts,  as  I  regard 
them,  are  briefly  these. 

In  1820,  Oersted  of  Copenhagen  made  the  celebrated 
discovery  which  connects  electricity  with  magnetism,  and 
immediately  afterward  the  acute  mind  of  WoUaston  per- 
ceived that  a  wire  carrying  a  current  ought  to  rotate 
round  its  own  axis  under  the  influence  of  a  magnetic 
pole.  In  1821  he  tried,  but  failed,  to  realize  this  result 
in  the  laboratory  of  the  Eoyal  Institution.  Faraday  was 
not  present  at  the  moment,  but  he  came  in  immediately 
afterward  and  heard  the  conversation  of  Wollaston  and 
Davy  about  the  experiment.  He  had  also  heard  a  rumor 
of  a  wager  that  Dr.  Wollaston  would  eventually  succeed. 

This  was  in  April.  In  the  autumn  of  the  same  year 
Faraday  wrote  a  history  of  electro-magnetism,  and  re- 
peated for  himself  the  experiments  which  he  described. 
It  was  while  thus  instructing  himself  that  he  succeeded 
in  causing  a  wire,  carrying  an  electric  current,  to  rotate 


428  FRAGMENTS  OF  SCIENCE 

round  a  magnetic  pole.  This  was  not  the  result  sought 
bj  WoUaston,  but  it  was  closely  related  to  that  result. 

The  strong  tendency  of  Faraday's  mind  to  look  upon 
the  reciprocal  actions  of  natural  forces  gave  birth  to  his 
greatest  discoveries;  and  we,  who  know  this,  should  be 
justified  in  concluding  that,  even  had  Wollaston  not  pre- 
ceded him,  the  result  would  have  been  the  same.  But  in 
judging  Davy  we  ought  to  transport  ourselves  to  his  time, 
and  carefully  exclude  from  our  thoughts  and  feelings  that 
noble  subsequent  life,  which  would  render  simply  impos- 
sible the  ascription  to  Faraday  of  anything  unfair.  It 
would  be  unjust  to  Davy  to  put  our  knowledge  in  the 
place  of  his,  or  to  credit  him  with  data  which  he  could 
not  have  possessed.  Eumor  and  fact  had  connected  the 
name  of  Wollaston  with  these  supposed  interactions  be- 
tween magnets  and  currents.  When,  therefore,  Faraday 
in  October  published  his  successful  experiment,  without 
any  allusion  to  Wollaston,  general,  though  really  un- 
grounded, criticism  followed.  I  say  ungrounded  because, 
first,  Faraday's  experiment  was  not  that  of  Wollaston, 
and  secondly,  Faraday,  before  he  published  it,  had  ac- 
tually called  upon  Wollaston,  and  not  finding  him  at 
home,  did  not  feel  himself  authorized  to  mention  his 
name. 

In  December,  Faraday  published  a  second  paper  on 
the  game  subject,  from  which,  through  a  misapprehension, 
the  name  of  Wollaston  was  also  omitted.  Warburton  and 
others  thereupon  affirmed  that  Wollaston's  ideas  had  been 
appropriated  without  acknowledgment,  and  it  is  plain  that 
Wollaston  himself,  though  cautious  in  his  utterance,  was 
also  hurt.  Censure  grew  till  it  became  intolerable.  "I 
hear,"  writes  Faraday  to  his  friend  Stodart,   "every  day 


LIFE   AND    LETTERS    OF   FARADAY  429 

more  and  more  of  these  sounds,  which,  though  only  whis- 
pers to  me,  are,  I  suspect,  spoken  aloud  among  scientific 
men."  He  might  have  written  explanations  and  defences, 
but  he  went  straighter  to  the  point.  He  wished  to  see  the 
principals  face  to  face — to  plead  his  cause  before  them 
personally.  There  was  a  certain  vehemence  in  his  desire 
to  do  this.  He  saw  Wollaston,  he  saw  Davy,  he  saw 
Warburton;  and  I  am  inclined  to  think  that  it  was  the 
irresistible  candor  and  truth  of  character  which  these 
viva  voce  defences  revealed,  as  much  as  the  defences  them- 
selves, that  disarmed  resentment  at  the  time. 

As  regards  Davy,  another  cause  of  dissension  arose  in 
1823.  In  the  spring  of  that  year  Faraday  analyzed  the 
hydrate  of  chlorine,  a  substance  once  believed  to  be  the 
element  chlorine,  but  proved  by  Davy  to  be  a  compound 
of  that  elenaent  and  water.  The  analysis  was  looked  over 
by  Davy,  who  then  and  there  suggested  to  Faraday  to 
heat  the  hydrate  in  a  closed  glass  tube.  This  was  done, 
the  substance  was  decomposed,  and  one  of  the  products 
of  decomposition  was  proved  by  Faraday  to  be  chlorine 
liquefied  by  its  own  pressure.  On  the  day  of  its  discov- 
ery he  communicated  this  result  to  Dr.  Paris.  Davy,  on 
being  informed  of  it,  instantly  liquefied  another  gas  in  the 
same  way.  Having  struck  thus  into  Faraday's  inquiry, 
ought  he  not  to  have  left  the  matter  in  Faraday's  hands? 
I  think  he  ought.  But,  considering  his  relation  to  both 
Faraday  and  the  hydrate  of  chlorine,  Davy,  I  submit,  may 
be  excused  for  thinking  differently.  A  father  is  not  al- 
ways wise  enough  to  see  that  his  son  has  ceased  to  be  a 
boy,  and  estrangement  on  this  account  is  not  rare;  nor 
was  Davy  wise  enough  to  discern  that  Faraday  had  passed 
the  mere  assistant  stage,  and  become  a  discoverer.     It  is 


430  FRAGMENTS   OF  SCIENCE 

now  liard  to  avoid  magnifying  this  error.  But  had  Fara- 
day died  or  ceased  to  work  at  this  time,  or  had  his  sub- 
sequent life  been  devoted  to  money-getting,  instead  of  to 
research,  would  anybody  now  dream  of  ascribing  jealousy 
to  Davy?  Assuredly  not.  Why  should  he  be  jealous? 
His  reputation  at  this  time  was  almost  without  a  parallel: 
his  glory  was  without  a  cloud.  He  had  added  to  his  other 
discoveries  that  of  Faraday,  and  after  having  been  his 
teacher  for  seven  years,  his  language  to  him  was  this:  "It 
gives  me  great  pleasure  to  hear  that  you  are  comfortable 
at  the  Eoyal  Institution,  and  I  trust  that  you  will  not 
only  do  something  good  and  honorable  for  yourself,  but 
also  for  science."  This  is  not  the  language  of  jealousy, 
potential  or  actual.  But  the  chlorine  business  introduced 
irritation  and  anger,  to  which,  and  not  to  any  ignobler 
motive,  Davy's  opposition  to  the  election  of  Faraday  to 
the  Royal  Society  is,  I  am  persuaded,  to  be  ascribed. 

These  matters  are  touched  upon  with  perfect  candor 
and  becoming  consideration  in  the  volumes  of  Dr.  Bence 
Jones:  but  in  "society"  they  are  not  always  so  handled. 
Here  a  name  of  noble  intellectual  associations  is  sur- 
rounded by  injurious  rumors  which  I  would  willingly 
scatter  forever.  The  pupil's  magnitude,  and  the  splen- 
dor of  his  position,  are  too  great  and  absolute  to  need  as 
a  foil  the  humiliation  of  his  master.  Brothers  in  intellect, 
Davy  and  Faraday,  however,  could  never  have  become 
brothers  in  feeling;  their  characters  were  too  unlike. 
Davy  loved  the  pomp  and  circumstance  of  fame;  Faraday 
the  inner  consciousness  that  he  had  fairly  won  renown. 
They  were  both  proud  men.  But  with  Davy  pride  pro- 
jected itself  into  the  outer  world;  while  with  Faraday  it 
became  a  steadying  and  dignifying  inward  force.     In  one 


LIFE   AND    LETTERS    OF  FARADAY  431 

great  particular  they  agreed.  Each  of  them  could  have 
turned  his  science  to  immense  commercial  profit,  but 
neither  of  them  did  so.  The  noble  excitement  of  research, 
and  the  delight  of  discovery,  constituted  their  reward.  I 
commend  them  to  the  reverence  which  great  gifts  greatly 
exercised  ought  to  inspire.  They  were  both  ours;  and 
through  the  coming  centuries  England  will  be  able  to 
point  with  just  pride  to  the  possession  of  such  men. 


The  first  volume  of  the  "Life  and  Letters"  reveals  to 
us  the  youth  who  was  to  be  father  to  the  man.  Skilful, 
aspiring,  resolute,  he  grew  steadily  in  knowledge  and 
in  power.  Consciously  or  unconsciously,  the  relation  of 
Action  to  Keaction  was  ever  present  to  Faraday's  mind. 
It  had  been  fostered  by  his  discovery  of  Magnetic  Eota- 
tions,  and  it  planted  in  him  more  daring  ideas  of  a  similar 
kind.  Magnetism  he  knew  could  be  evoked  by  electricity, 
and  he  thought  that  electricity,  in  its  turn,  ought  to  be 
capable  of  evolution  by  magnetism.  On  August  29,  1831, 
his  experiments  on  this  subject  began.  He  had  been  for- 
tified by  previous  trials,  which,  though  failures,  had  be- 
gotten instincts  directing  him  toward  the  truth.  He,  like 
every  strong  worker,  might  at  times  miss  the  outward  ob- 
ject, but  he  always  gained  the  inner  light,  education,  and 
expansion.  Of  this  Faraday's  life  was  a  constant  illustra- 
tion. By  November  he  had  discovered  and  colligated  a 
multitude  of  the  most  wonderful  and  unexpected  phe- 
nomena. He  had  generated  currents  by  currents;  cur- 
rents by  magnets,  permanent  and  transitory ;  and  he  after- 
ward generated  currents  by  the  earth  itself.  Arago's 
'*Magnetism  of    Rotation,"    which    had    for  years   offered 


432  FRAGMENTS   OF  SCIENCE 

itself  as  a  challenge  to  the  best  scientific  intellects  of 
Europe,  now  fell  into  his  hands.  It  proved  to  be  a  beau- 
tiful, but  still  special,  illustration  of  the  great  principle 
of  Magneto- electric  Induction.  Nothing  equal  to  this 
latter,  in  the  way  of  pure  experimental  inquiry,  had  pre- 
viously been  achieved. 

Electricities  from  various  sources  were  next  examined, 
and  their  differences  and  resemblances  revealed.  He  thus 
assured  himself  of  their  substantial  identity.  He  then  took 
up  Conduction,  and  gave  many  striking  illustrations  of 
the  influence  of  Fusion  on  Conducting  Power.  Eenounc- 
ing  professional  work,  from  which  at  this  time  he  might 
have  derived  an  income  of  many  thousands  a  year,  he 
poured  his  whole  momentum  into  his  researches.  He  was 
long  entangled  in  Electro-chemistry,  The  light  of  law 
was  for  a  time  obscured  by  the  thick  umbrage  of  novel 
facts;  but  he  finally  emerged  from  his  researches  with  the 
great  principle  of  Definite  Electro-chemical  Decomposition 
in  his  hands.  If  his  discovjery  of  Magneto- electricity  may 
be  ranked  with  that  of  the  pile  by  Volta,  this  new  discov- 
ery may  almost  stand  beside  that  of  Definite  Combining 
Proportions  in  Chemistry.  He  passed  on  to  Static  Elec- 
tricity— its  Conduction,  Induction,  and  Mode  of  Propaga- 
tion. He  discovered  and  illustrated  the  principle  of  In- 
ductive Capacity;  and,  turning  to  theory,  he  asked  himself 
how  electrical  attractions  and  repulsions  are  transmitted. 
Are  they,  like  gravity,  actions  at  a  distance,  or  do  they 
require  a  medium  ?  If  the  former,  then,  like  gravity,  they 
will  act  in  straight  lines;  if  the  latter,  then,  like  sound 
or  light,  they  may  turn  a  comer.  Faraday  held — and  his 
views  are  gaining  ground — that  his  experiments  proved 
the  fact  of  curvilinear  propagation,  and  hence  the  opera- 


LIFE  AND   LETTERS  OF  FARADAY  433 

tion  of  a  medium.  Others  dened  this;  but  none  can 
deny  the  profound  and  philosophic  character  of  his  lead- 
ing thought.  *  The  first  volume  of  the  Kesearches  contains 
all  the  papers  here  referred  to. 

Faraday  had  heard  it  stated  that  henceforth  physical 
discoveries  would  be  made  solely  by  the  aid  of  mathemat- 
ics; that  we  had  our  data,  and  needed  only  to  work  de- 
ductively. Statements  of  a  similar  character  crop  out 
from  time  to  time  in  our  day.  They  arise  from  an  im- 
perfect acquaintance  with  the  nature,  present  condition, 
and  prospective  vastness  of  the  field  of  physical  inquiry. 
The  tendency  of  natural  science  doubtless  is  to  bring  all 
physical  phenomena  under  the  dominion  of  mechanical 
laws;  to  give  them,  in  other  words,  mathematical  expres- 
sion. But  our  approach  to  this  result  is  asymptotic;  and 
for  ages  to  come — possibly  for  all  the  ages  of  the  human 
race — Nature  will  find  room  for  both  the  philosophical  ex- 
perimenter and  the  mathematician.  Faraday  entered  his 
protest  against  the  foregoing  statement  by  labelling  his 
investigations  **  Experimental  Eesearches  in  Electricity. ' ' 
They  were  completed  in  1854,  and  three  volumes  of  them 
have  been  published.  For  the  sake  of  reference,  he  num- 
bered every  paragraph,  the  last  number  being  3,862.  In 
1859  he  collected  and  published  a  fourth  volume  of  pa- 
pers, under  the  title,  **  Experimental  Eesearches  in  Chem- 
istry and  Physics."  Thus  did  this  apostle  of  experiment 
illustrate  its  power  and  magnify  his  office. 

The  second  volume  of  the  Researches  embraces  mem- 


>  In  a  very  remarkable  paper  published  in  PoggendorfE's  "Annalen"  for 
1857,  Werner  Siemens  accepts  and  develops  Faraday's  theory  of  Molecular 
Induction. 

Science— V— 19 


434  FRAGMENTS    OF  SCIENCE 

oirs  on  the  Electricity  of  the  Grymnotus;  on  the  Source 
of  Power  in  the  Voltaic  Pile;  on  the  Electricity  evolved 
by  the  Friction  of  Water  and  Steam,  in  which  the  phe- 
nomena and  principles  of  Sir  William  Armstrong's  Hydro- 
electric machine  are  described  and  developed;  a  paper  on 
Magnetic  flotations,  and  Faraday's  letters  in  relation  to 
the  controversy  it  aroused.  The  contribution  of  most  per- 
manent value  here  is  that  on  the  Source  of  Power  in  the 
Voltaic  Pile.  By  it  the  Contact  Theory,  pure  and  simple, 
was  totally  overthrown,  and  the  necessity  of  chemical 
action  to  the  maintenance  of  the  current  demonstrated. 

The  third  volume  of  the  Researches  opens  with  a 
memoir  entitled  "The  Magnetization  of  Light,"  and  the 
*' Illumination  of  Magnetic  Lines  of  Force. '*  It  is  diffi- 
cult even  now  to  affix  a  definite  meaning  to  this  title;  but 
the  discovery  of  the  rotation  of  the  plane  of  polarization, 
which  it  announced,  seems  pregnant  with  great  results. 
The  writings  of  William  Thomson  on  the  theoretic  aspects 
of  the  discovery;  the  excellent  electro- dynamic  measure- 
ments of  Wilhelm  Weber,  which  are  models  of  experi- 
mental completeness  and  skill;  Weber's  labors  in  con- 
junction with  his  lamented  friend  Kohlrausch — above  all, 
the  researches  of  Clerk  Maxwell  on  the  Electro-magnetic 
Theory  of  Light — point  to  that  wonderful  and  mysterious 
medium,  which  is  the  vehicle  of  light  and  radiant  heat,  as 
the  probable  basis  also  of  magnetic  and  electric  phenom- 
ena. The  hope  of  such  a  connection  was  first  raised  by 
the  discovery  here  referred  to.*     Faraday  himself  seemed 


*  A  letter  addressed  to  me  by  Professor  Weber  on  March  18  last,  contains 
the  following  reference  to  the  connection  here  mentioned:  "Die  Hoffnung  einer 
solchen  Combination  ist  durch  Faraday's  Entdeckung  der  Drehung  der  Polar- 
isationsebene  durch  magnetische  Directionskraft  zuerst,  und  sodann  durch  die 


LIFE   AND   LETTERS   OF  FARADAY  436 

to  cling  with  particular  affection  to  this  discovery.  H« 
felt  that  there  was  more  in  it  than  he  was  able  to  unfold. 
He  predicted  that  it  would  grow  in  meaning  with  the 
growth  of  science.  This  it  has  done;  this  it  is  doing 
now.  Its  right  interpretation  will  probably  mark  an 
epoch  in  scientific  history. 

Rapidly  following  it  is  the  discovery  of  Diamagnetism, 
or  the  repulsion  of  matter  by  a  magnet.  Brugmans  had 
shown  that  bismuth  repelled  a  magnetic  needle.  Here  ho 
stopped.  Le  Bailliff  proved  that  antimony  did  the  same. 
Here  he  stopped.  Seebeck,  Becquerel,  and  others,  also 
touched  the  discovery.  These  fragmentary  gleams  excited 
a  momentary  curiosity  and  were  almost  forgotten,  when 
Faraday  independently  alighted  on  the  same  facts;  and/ 
instead  of  stopping,  made  them  the  inlets  to  a  new  and 
vast  region  of  research.  The  value  of  a  discovery  is  to 
be  measured  by  the  intellectual  action  it  calls  forth;  and 
it  was  Faraday's  good  fortune  to  strike  such  lode?  of 
scientific  truth  as  give  occupation  to  some  of  the  best 
intellects  of  our  age. 

The  salient  quality  of  Faraday's  scientific  character 
reveals  itself  from  beginning  to  end  of  these  volumes;  a 
union  of  ardor  and  patience — the  one  prompting  the  at- 
tack, the  other  holding  him  on  to  it,  till  defeat  was  final 
or  victory  assured.  Certainty  in  one  sense  or  the  other 
was  necessary  to  his  peace  of  mind.  The  right  method  of 
investigation   is   perhaps   incommunicable;    it   depends   on 


Uebereinstimmung  derjenigen  Geschwindigkeit,  welche  das  Verhaltniss  der 
electro-dynaraischen  Einheit  zur  electro-statischen  ausdriickt,  mit  der  Cteachwin- 
digkeit  des  Lichte  angeregt  worden;  und  mir  scheint  von  alien  Yersuchen^ 
welche  zur  Verwirklichung  dieser  Hoffnung  gemacht  worden  sind,  daa  von 
Herrn  Maxwell  demachte  am  erfolgreichsten." 


436  •  FRAGMENTS   O^   SCIENCE 

the  individual  rather  than  on  the  system,  and  the  mark 
is  missed  when  Faraday's  researches  are  pointed  to  as 
merely  illustrative  of  the  power  of  the  inductive  philoso- 
phy. The  brain  may  be  filled  with  that  philosophy;  but 
without  the  energy  and  insight  which  this  man  possessed, 
and  which  with  him  were  personal  and  distinctive,  we 
should  never  rise  to  the  level  of  his  achievements.  His 
power  is  that  of  individual  genius,  rather  than  of  philo- 
sophic method;  the  energy  of  a  strong  soul  expressing 
itself  after  its  own  fashion,  and  acknowledging  no  medi- 
ator between  it  and  Nature. 

The  second  volume  of  the  *'Life  and  Letters,"  like  the 
first,  is  a  historic  treasury  as  regards  Faraday's  work  and 
'character,  and  his  scientific  and  social  relations.  It  con- 
tains letters  from  Humboldt,  Herschel,  Hachette,  De  la 
Rive,  Dumas,  Liebig,  Melloni,  Becquerel,  Oersted,  Pliicker, 
Du  Bois-Eeymond,  Lord  Melbourne,  Prince  Louis  Napo- 
leon, and  many  other  distinguished  men.  I  notice,  with 
particular  pleasure,  a  letter  from  Sir  John  Herschel,  in 
reply  to  a  sealed  packet  addressed  to  him  by  Faraday, 
but  which  he  had  permission  to  open  if  he  pleased.  The 
packet  referred  to  one  of  the  many  unfulfilled  hopes  which 
spring  up  in  the  minds  of  fertile  investigators: 

*'G-o  on  and  prosper,  'from  strength  to  strength,'  like 
a  victor  marching  with  assured  step  to  further  conquests; 
and  be  certain  that  no  voice  will  join  more  heartily  in. 
the  peans  that  already  begin  to  rise,  and  will  speedily 
swell  into  a  shout  of  triumph,  astounding  even  to  your- 
self, than  that  of  J.  F.  W.  Herschel." 

Faraday's  behavior  to  Melloni,  in  1835,  merits  a  word 
of  notice.  The  young  man  was  a  political  exile  in  Paris. 
He  had  newly  fashioned  and  applied  the  thermo-electric 


LIFE   AND   LETTERS   OF  FARADAY  437 

pile,  and  had  obtained  with  it  results  of  the  greatest  im- 
portance. But  they  were  not  appreciated.  With  the  sick- 
ness of  disappointed  hope,  Melloni  waited  for  the  report 
of  the  Commissioners,  appointed  by  the  Academy  of  Sci- 
ences to  examine  the  Primier.  At  length  he  published 
his  researches  in  the  "Annales  de  Chimie."  They  thus 
fell  into  the  hands  of  Faraday,  who,  discerning  at  once 
their  extraordinary  merit,  obtained  for  their  author  the 
Kumford  Medal  of  the  Eoyal  Society.  A  sum  of  money 
always  accompanies  this  medal;  and  the  pecuniary  help 
was,  at  this  time,  even  more  essential  than  the  mark 
of  honor  to  the  young  refugee.  Melloni 's  gratitude  was 
boundless: 

"Et  vous,  monsieur,**  he  writes  to  Faraday,  "qui  ap- 
partenez  a  une  soci^t^  h  laquelle  je  n'avais  rien  offert, 
vous  qui  me  connaissiez  k  peine  de  nom;  vous  n'avez  pas 
demand^  si  j'avais  des  ennemis  faibles  ou  puissants,  ni 
calculi  quel  en  ^tait  le  nombre;  mais  vous  avez  parl^ 
pour  I'opprime  Stranger,  pour  celui  qui  n'avait  pas  le 
moindre  droit  a  tant  de  bienveillance,  et  vos  paroles  ont 
^t^  accueillies  favorablement  par  des  collogues  conscien- 
cieux!  Je  reconnais  bien  la  des  hommes  dignes  de  leur 
noble  mission,  les  veritable  representants  de  la  science 
d'un  pays  libre  et  g^nereux. " 

Within  the  prescribed  limits  of  this  article  it  would  be 
impossible  to  give  even  the  slenderest  summary  of  Fara- 
day's correspondence,  or  to  carve  from  it  more  than  the 
merest  fragments  of  his  character.  His  letters,  written  to 
Lord  Melbourne  and  others  in  1836,  regarding  his  pen- 
sion, illustrate  his  uncompromising  independence.  The 
Prime  Minister  had  offended  him,  but  assuredly  the  apol- 
ogy demanded  and  given  was  complete.     I  think  it  cer- 


438  FRAGMENTS   OF  SCIENCE 

tain  that,  notwithstanding  the  very  full  account  of  this 
transaction  given  by  Dr.  Bence  Jones,  motives  and  influ- 
ences were  at  work  which  even  now  are  not  entirely  re- 
vealed. The  minister  was  bitterly  attacked,  but  he  bore 
the  censure  of  the  press  with  great  dignity.  Faraday, 
while  he  disavowed  having  either  directly  or  indirectly 
furnished  the  matter  of  those  attacks,  did  not  publicly 
exonerate  the  Premier.  The  Hon.  Caroline  Fox  had 
proved  herself  Faraday's  ardent  friend,  and  it  was  she 
who  had  healed  the  breach  between  the  philosopher  and 
the  minister.  She  manifestly  thought  that  Faraday  ought 
to  have  come  forward  in  Lord  Melbourne's  defence,  and 
there  is  a  flavor  of  resentment  in  one  of  her  letters  to 
him  on  the  subject.  No  doubt  Faraday  had  good  grounds 
for  his  reticence,  but  they  are  to  me  unknown. 

In  1841  his  health  broke  down  utterly,  and  he  went 
to  Switzerland  with  his  wife  and  brother-in-law.  His 
bodily  vigor  soon  revived,  and  he  accomplished  feats  of 
walking  respectable  even  for  a  trained  mountaineer.  The 
published  extracts  from  his  Swiss  journal  contain  many 
beautiful  and  touching  allusions.  Amid  references  to  the 
tints  of  the  Jungfrau,  the  blue  rifts  of  the  glaciers,  and 
the  noble  Kiesen  towering  over  the  Lake  of  Thun,  we 
come  upon  the  charming  little  scrap  which  I  have  else- 
where quoted:  "Clout-nail  making  goes  on  here  rather 
considerably,  and  is  a  very  neat  and  pretty  operation  to 
observe.  I  love  a  smith's  shop  and  anything  relating  to 
smithery.  My  father  was  a  smith."  This  is  from  his 
journal;  but  he  is  unconsciously  speaking  to  somebody 
— ^perhaps  to  the  world. 

His  description  of  the  Staubbach,  Giessbach,  and  of 
the  scenic  effects  of  sky  and  mountain,  are  all  fine  and 


LIFE   AND    LETTERS    OF   FARADAY  439 

sympathetic.  But  amid  it  all,  and  in  reference  to  it  all, 
he  tells  his  sister  that  "true  enjoyment  is  from  within, 
not  from  without."  In  those  days  Agassiz  was  living 
under  a  slab  of  gneiss  on  the  glacier  of  the  Aar.  Fara- 
day met  Forbes  at  the  Grimsel,  and  arranged  with  him 
an  excursion  to  the  "Hotel  des  Neuch^telois" ;  but  indis- 
position put  the  project  out. 

From  the  Fort  of  Ham,  in  1843,  Faraday  received  a 
letter  addressed  to  him  by  Prince  Louis  Napoleon  Bona- 
parte. He  read  this  letter  to  me  many  years  ago,  and  the 
desire,  shown  in  various  ways  by  the  French  Emperor,  to 
turn  modern  science  to  account,  has  often  reminded  me 
of  it  since.  At  the  age  of  thirty-five  the  prisoner  of  Ham 
speaks  of  "rendering  his  captivity  less  sad  by  studying 
the  great  discoveries"  which  science  owes  to  Faraday; 
and  he  asks  a  question  which  reveals  his  cast  of  thought 
at  the  time:  "What  is  the  most  simple  combination  to 
give  to  a  voltaic  battery,  in  order  to  produce  a  spark 
capable  of  setting  fire  to  powder  under  water  or  under 
ground?"  Should  the  necessity  arise,  the  French  Em 
peror  will  not  lack  at  the  outset  the  best  appliances  of 
modern  science;  while  we,  I  fear,  shall  have  to  learn  the 
magnitude  of  the  resources  we  are  now  neglecting  amid 
the  pangs  of  actual  war.* 

One  turns  with  renewed  pleasure  to  Faraday's  letters 
to  his  wife,  published  in  the  second  volume.  Here  surely 
the  loving  essence  of  the  man  appears  more  distinctly  than 


*  The  "science"  has  since  been  applied,  with  astonishing  effect,  by  those 
who  had  studied  it  far  more  thoroughly  than  the  Emperor  of  the  French.  We 
also,  I  am  happy  to  think,  have  improved  the  time  since  the  above  words  wer« 
written  [1878]. 


440  FRAGMENTS    OF  SCIENCS 

anywhere  else.  From  the  house  of  Dr.  Percy,  in  Birming- 
ham, he  writes  thus: 

"Here — even  here — the  moment  I  leave  the  table,  I 
wish  I  were  with  you  IN  quiet.  Oh,  what  happiness 
is  ours!  My  runs  into  the  world  in  this  way  only  serve 
to  make  me  esteem  that  happiness  the  more." 

And  again: 

"We  have  been  to  a  grand  conversazione  in  thetown- 
hali,  and  I  have  now  returned  to  my  room  to  talk  with 
you,  as  the  pleasantest  and  happiest  thing  that  I  can  do. 
Nothing  rests  me  so  much  as  communion  with  you.  I 
feel  it  even  now  as  I  write,  and  catch  myself  saying  the 
words  aloud  as  I  write  them." 

Take  this,  moreover,  as  indicative  of  his  love  for 
Nature: 

"After  writing,  I  walk  out  in  the  evening  hand  in 
hand  with  my  dear  wife  to  enjoy  the  sunset;  for  to  me 
who  love  scenery,  of  all  that  I  have  seen  or  can  see,  there 
is  none  surpasses  that  of  heaven.  A  glorious  sunset  brings 
with  it  a  thousand  thoughts  that  delight  me." 

Of  the  numberless  lights  thrown  upon  him  by  the 
"Life  and  Letters,"  some  fall  upon  his  religion.  In  a 
letter  to  Lady  Lovelace,  he  describes  himself  as  belonging 
to  "a  very  small  and  despised  sect  of  Christians,  known, 
if  known  at  all,  as  Sandemanians^  and  our  hope  is  founded 
on  the  faith  that  is  in  Christ."  He  adds:  "I  do  not  think 
it  at  all  necessary  to  tie  the  study  of  the  natural  sciences 
and  religion  together,  and,  in  my  intercourse  with  my 
fellow- creatures,  that  which  is  religious,  and  that  which 
is  philosophical,  have  ever  been  two  distinct  things. "  He 
saw  clearly  the  danger  of  quitting  his  moorings,  and  his 
science  acted  indirectly  as  the  safeguard  of  his  faith.     For 


LIFE   AND    LETTERS    OF  FARADAY  441 

his  investigations  so  filled  his  mind  as  to  leave  no  room 
for  sceptical  questionings,  thus  shielding  from  the  assaults 
of  philosophy  the  creed  of  his  youth.  His  religion  was 
constitutional  and  hereditary.  It  was  implied  in  the  ed- 
dies of  his  blood  and  in  the  tremors  of  his  brain;  and, 
however  its  outward  and  visible  form  might  have  changed, 
Faraday  would  still  have  possessed  its  elemental  constitu- 
ents— awe,  reverence,  truth,  and  love. 

It  is  worth  inquiring  how  so  profoundly  religious  a 
mind,  and  so  great  a  teacher,  would  be  likely  to  regard 
our  present  discussions  on  the  subject  of  education.  Far- 
aday would  be  a  *' secularist"  were  he  now  alive.  He  had 
no  sympathy  with  those  who  contemn  knowledge  unless 
it  be  accompanied  by  dogma.  A  lecture  delivered  before 
the  City  Philosophical  Society  in  1818,  when  he  was 
twenty -six  years  of  age,  expresses  the  views  regarding 
education  which  he  entertained  to  the  end  of  his  life. 
*' First,  then,"  he  says,  "all  theological  considerations  are 
banished  from  the  society,  and  of  course  from  my  re- 
marks; and  whatever  I  may  say  has  no  reference  to  a 
future  state,  or  to  the  means  which  are  to  be  adopted  in 
this  world  in  anticipation  of  it.  Next,  I  have  no  inten- 
tion of  substituting  anything  for  religion,  but  I  wish  to  take 
that  part  of  human  nature  which  is  independent  of  it. 
Morality,  philosophy,  'commerce,  the  various  institutions 
and  habits  of  society,  are  independent  of  religion,  and 
may  exist  either  with  or  without  it.  They  are  always  the 
same,  and  can  dwell  alike  in  the  breasts  of  those  who, 
from  opinion,  are  entirely  opposed  in  the  set  of  principles 
they  include  in  the  term  religion,  or  in  those  who  have 
none. 

"To  discriminate  more  closely,  if  possible,  I  will  ob- 


442  FRAGMENTS   OF  SCIENCE 

serve  that  we  have  no  right  to  judge  religions  opinions; 
but  the  human  nature  of  this  evening  is  that  part  of  man 
which  we  have  a  right  to  judge.  And  I  think  it  will  be 
found  on  examination,  that  this  humanity — as  it  may  per- 
haps be  called — will  accord  with  what  I  have  before  de- 
scribed as  being  in  our  own  hands  so  improvable  and 
perfectible. ' ' 

In  an  old  journal  I  find  the  following  remarks  on  one 
of  my  earliest  dinners  with  Faraday:  "At  two  o'clock  he 
came  down  for  me.  He,  his  niece,  and  myself,  formed 
the  party.  'I  never  give  dinners,'  he  said.  'I  don't  know 
how  to  give  dinners,  and  I  never  dine  out.  But  I  should 
not  like  my  friends  to  attribute  this  to  a  wrong  cause. 
I  act  thus  for  the  sake  of  securing  time  for  work,  and  not 
through  religious  motives,  as  some  imagine.'  He  said 
grace.  I  am  almost  ashamed  to  call  his  prayer  a  'saying 
of  grace.'  In  the  language  of  Scripture,  it  might  be  de- 
scribed as  the  petition  of  a  son  into  whose  heart  God  had 
sent  the  Spirit  of  His  Son,  and  who  with  absolute  trust 
asked  a  blessing  from  his  father.  We  dined  on  roast  beef, 
Yorkshire  pudding,  and  potatoes;  drank  sherry,  talked 
of  research  and  its  requirements,  and  of  his  habit  of  keep- 
ing himself  free  from  the  distractions  of  society.  He  was 
bright  and  joyful — boy  like,  in  fact,  though  he  is  now 
sixty-two.  His  work  excites  admiration,  but  contact  with 
him  warms  and  elevates  the  heart.  Here,  surely,  is  a 
strong  man.  I  love  strength;  but  let  me  not  forget  the 
example  of  its  union  with  modesty,  tenderness,  and  sweet- 
ness, in  the  character  of  Faraday. ' ' 

Faraday's  progress  in  discovery,  and  the  salient  points 
of  his  character,  are  well  brought  out  by  the  wise  choice 
of  letters  and  extracts  published  in  the  volumes  before  us. 


LIFE   AND    LETTERS    OF   FARADAY  443 

I  will  not  call  the  labors  of  the  biographer  final.  So  great 
a  character  will  challenge  reconstruction.  In  the  coming 
time  some  sympathetic  spirit,  with  the  requisite  strength, 
knowledge,  and  solvent  power,  will,  I  doubt  not,  render 
these  materials  plastic,  give  them  more  perfect  organic 
form,  and  send  through  them,  with  less  of  interruption, 
the  currents  of  Faraday's  life.  *'He  was  too  good  a  man," 
writes  his  present  biographer,  *'for  me  to  estimate  rightly, 
and  too  great  a  philosopher  for  me  to  understand  thor- 
oughly." That  may  be:  but  the  reverent  affection  to 
which  we  owe  the  discovery,  selection,  and  arrangement 
of  the  materials  here  placed  before  us  is  probably  a  surer 
guide  than  mere  literary  skill.  The  task  of  the  artist  who 
may  wish  m  future  times  to  reproduce  the  real  though 
unobtrusive  grandeur,  the  purity,  beauty,  and  childlike 
simplicity  of  him  whom  we  have  lost,  will  find  his  chief 
treasury  already  provided  for  him  by  Dr.  Bence  Jones's 
labor  of  love. 


XIX 

THE   COPLEY   MEDALIST   OF  1870 

THIE1 Y  years  ago  Electro -magnetism  was  looked  to 
as  a  motive  power  which  might  possibly  compete 
with  steam.  In  centres  of  industry,  such  as  Man- 
chester, attempts  to  investigate  and  apply  this  power  were 
numerous.  This  is  shown  by  the  scientific  literature  of  the 
time.  Among  others  Mr.  James  Prescot  Joule,  a  resident 
of  Manchester,  took  up  the  subject,  and,  in  a  series  of 
papers  published  in  Sturgeon's  "Annals  of  Electricity" 
between  1839  and  1841,  described  various  attempts  at  the 
construction  and  perfection  of  electro-magnetic  engines. 
The  spirit  in  which  Mr.  Joule  pursued  these  inquiries  is 
revealed  in  the  following  extract:  "I  am  particularly  anx- 
ious," he  says,  "to  communicate  any  new  arrangement  in 
order,  if  possible,  to  forestall  the  monopolizing  designs 
of  those  who  seem  to  regard  this  most  interesting  subject 
merely  in  the  light  of  pecuniary  speculation."  He  was 
naturally  led  to  investigate  the  laws  of  electro-magnetic 
attractions,  and  in  1840  he  announced  the  important  prin- 
ciple that  the  attractive  force  exerted  by  two  electro-mag- 
nets, or  by  an  electro- magnet  and  a  mass  of  annealed  iron, 
is  directly  proportional  to  the  square  of  the  strength  of  the 
magnetizing  current;  while  the  attraction  exerted  between 
an  electro-magnet  and  the  pole  of  a  permanent  steel  mag- 
net varies  simply  as  the  strength  of  the  current.  These 
(444) 


THE   COPLEY  MEDALIST  OF  1870  445 

investigations  were  conducted  independently  of,  though 
a  little  subsequently  to,  the  celebrated  inquiries  of  Henry, 
Jacobi,  and  Lenz  and  Jacobi,  on  the  same  subject. 

On  December  17,  1840,  Mr.  Joule  communicated  to  the 
Royal  Society  a  paper  on  the  production  of  heat  by  Vol- 
taic electricity.  In  it  he  announced  the  law  that  the  calo- 
rific effects  of  equal  quantities  of  transmitted  electricity  are 
proportional  to  the  resistance  overcome  by  the  current, 
whatever  may  be  the  length,  thickness,  shape,  or  char- 
acter of  the  metal  which  closes  the  circuit;  and  also  pro- 
portional to  the  square  of  the  quantity  of  transmitted 
electricity.  This  is  a  law  of  primary  importance.  In 
another  paper,  presented  to,  but  declined  by,  the  Royal 
Society,  he  confirmed  this  law  by  new  experiments,  and 
materially  extended  it.  He  also  executed  experiments  on 
the  heat  consequent  on  the  passage  of  Voltaic  electricity 
through  electrolytes,  and  found,  in  all  cases,  that  the  heat 
evolved  by  the  proper  action  of  any  Voltaic  current  is  pro- 
portional to  the  square  of  the  intensity  of  that  current, 
multiplied  by  the  resistance  to  conduction  which  it  expe- 
riences. From  this  law  he  deduced  a  number  of  conclu- 
sions of  the  highest  importance  to  electro-chemistry. 

It  was  during  these  inquiries,  which  are  marked 
throughout  by  rare  sagacity  and  originality,  that  the  great 
idea  of  establishing  quantitative  relations  between  Me- 
chanical Energy  and  Heat  arose  and  assumed  definite  form 
in  his  mind.  In  1843  Mr.  Joule  read  before  the  meeting 
of  the  British  Association  at  Cork  a  paper  ' '  On  the  Calo- 
rific Effects  of  Magneto-Electricity,  and  on  the  Mechanical 
Value  of  Heat. ' '  Even  at  the  present  day  this  memoir  is 
tough  reading,  and  at  the  time  it  was  written  it  must  have 
appeared  hopelessly  entangled.     This,  I  should  think,  was 


446  FRAGMENTS   OF  SCIENCE 

the  reason  why  Faraday  advised  Mr.  Joule  not  to  submit 
the  paper  to  the  Koyal  Society.  But  its  drift  and  results 
are  summed  up  in  these  memorable  words  by  its  author, 
written  some  time  subsequently:  **In  that  paper  it  was 
demonstrated,  experimentally,  that  the  mechanical  power 
exerted  in  turning  a  magneto-electric  machine  is  converted 
into  the  heat  evolved  by  the  passage  of  the  currents  of 
induction  through  its  coils;  and,  on  the  other  hand,  that 
the  motive  power  of  the  electro-magnetic  engine  is  ob- 
tained at  the  expense  of  the  heat  due  to  the  chemical 
reaction  of  the  battery  by  which  it  is  worked. " '  It  is 
needless  to  dwell  upon  the  weight  and  importance  of  this 
statement. 

Considering  the  imperfections  incidental  to  a  first  deter- 
mination, it  is  not  surprising  that  the  *' mechanical  values 
of  heat,"  deduced  from  the  different  series  of  experiments 
published  in  1843,  varied  widely  from  each  other.  The 
lowest  limit  was  587  and  the  highest  1,026  foot-pounds, 
for  1°  Fahr.   of  temperature. 

One  noteworthy  result  of  his  inquiries,  which  was 
pointed  out  at  the  time  by  Mr.  Joule,  had  reference  to 
the  exceedingly  small  fraction  of  the  heat  actually  con- 
verted into  useful  effect  in  the  steam-engine.  The  thoughts 
of  the  celebrated  Julius  Eobert  Mayer,  who  was  then  en- 
gaged in  Germany  upon  the  same  question,  had  moved 
independently  in  the  same  groove;  but  to  his  labors  due 
reference  will  be  made  on  a  future  occasion.''  In  the 
memoir  now  referred  to,  Mr.  Joule  also  announced  that 
lie  had  proved  heat  to  be  evolved  during  the  passage  of 
water  through  narrow  tubes;    and  he  deduced  from  these 

*  "Phil.  Mag.,"  May,  1845.  •  See  the  next  Fragment. 


THE  COPLEY   MEDALIST  OF  1870  447 

experiments  an  equivalent  of  770  foot-pounds,  a  figure 
remarkably  near  the  one  now  accepted.  A  detached  state- 
ment regarding  the  origin  and  convertibility  of  animal  heat 
strikingly  illustrates  the  penetration  of  Mr.  Joule,  and  his 
mastery  of  principles,  at  the  period  now  referred  to.  A 
friend  had  mentioned  to  him  Haller's  hypothesis,  that 
animal  heat  might  arise  from  the  friction  of  the  blood 
in  the  veins  and  arteries.  *'It  is  unquestionable,"  writes 
Mr.  Joule,  *'that  heat  is  produced  by  such  friction;  but 
it  must  be  understood  that  the  mechanical  force  expended 
in  the  friction  is  a  part  of  the  force  of  affinity  which 
causes  the  venous  blood  to  unite  with  oxygen,  so  that  the 
whole  heat  of  the  system  must  still  be  referred  to  the 
chemical  changes.  But  if  the  animal  were  engaged  in 
turning  a  piece  of  machinery,  or  in  ascending  a  moun- 
tain, I  apprehend  that  in  proportion  to  the  muscular  effort 
put  forth  for  the  purpose,  a  dimi7iution  of  the  heat  evolved 
in  the  system  by  a  given  chemical  action  would  be  expe- 
rienced."  The  italics  in  this  memorable  passage,  written, 
it  is  to  be  remembered,  in  1843,  are  Mr.  Joule's  own. 

The  concluding  paragraph  of  this  British  Association 
paper  equally  illustrates  his  insight  and  precision  regard- 
ing the  nature  of  chemical  and  latent  heat.  *'I  had,"  he 
writes,  "endeavored  to  prove  that  when  two  atoms  com- 
bine together,  the  heat  evolved  is  exactly  that  which  would 
have  been  evolved  by  the  electrical  current  due  to  the 
chemical  action  taking  place,  and  is  therefore  proportional 
to  the  intensity  of  the  chemical  force  causing  the  atoms 
to  combine.  I  now  venture  to  state  more  explicitly  that 
it  is  not  precisely  the  attraction  of  affinity,  but  rather  the 
mechanical  force  expended  by  the  atoms  in  falling  toward 
one  another,   which   determines   the  intensity  of  the  cur- 


448  FRAGMENTS   OF  SCIENCE 

rent,  and,  consequently,  tlie  quantity  of  heat  evolved; 
so  that  we  have  a  simple  hypothesis  by  which  we  may 
explain  why  heat  is  evolved  so  freely  in  the  combination 
of  gases,  and  by  which  indeed  we  may  account  *  latent 
heat'  as  a  mechanical  power,  prepared  for  action,  as  a 
watch-spring  is  when  wound  up.  Suppose,  for  the  sake 
of  illustration,  that  8  lbs.  of  oxygen  and  1  lb.  of  hydrogen 
were  presented  to  one  another  in  the  gaseous  state,  and 
then  exploded ;  the  heat  evolved  would  be  about  1°  Fahr. 
in  60,000  lbs.  of  water,  indicating  a  mechanical  force, 
expended  in  the  combination,  equal  to  a  weight  of  about 
50,000,000  lbs.  raised  to  the  height  of  one  foot.  Now  if 
the  oxygen  and  hydrogen  could  be  presented  to  each  other 
in  a  liquid  state,  the  heat  of  combination  would  be  less 
than  before,  because  the  atoms  in  combining  would  fall 
through  less  space."  No  words  of  mine  are  needed  to 
point  out  the  commanding  grasp  of  molecular  physics,  in 
their  relation  to  the  mechanical  theory  of  heat,  implied 
by  this  statement. 

Perfectly  assured  of  the  importance  of  the  principle 
which  his  experiments  aimed  at  establishing,  Mr.  Joule 
did  not  rest  content  with  results  presenting  such  discrep- 
ancies as  those  above  referred  to.  He  resorted  in  1844  to 
entirely  new  methods,  and  made  elaborate  expe'riments 
on  the  thermal  changes  produced  in  air  during  its  expan- 
sion: first,  against  a  pressure,  and  therefore  performing 
work;  secondly,  against  no  pressure,  and  therefore  per- 
forming no  work.  He  thus  established  anew  the  relation 
between  the  heat  consumed  and  the  work  done.  From 
^ye  different  series  of  experiments  he  deduced  five  differ- 
ent mechanical  equivalents;  the  agreement  between  them 
being  far  greater  than   that   attained   in   his   first  experi- 


THE   COPLEY   MEDALIST  OF  1870  449 

ments.  The  mean  of  them  was  802  foot-pounds.  From 
experiments  with  water  agitated  by  a  paddle-wheel,  he 
deduced,  in  1845,  an  equivalent  of  890  foot-pounds.  In 
1847  he  again  operated  upon  water  and  sperm -oil,  agitated 
them  bj  a  paddle-wheel,  determined  their  elevation  of 
temperature,  and  the  mechanical  power  which  produced 
it.  From  the  one  he  derived  an  equivalent  of  781*5  foot- 
pounds; from  the  other  an  equivalent  of  782*1  foot-pounds. 
The  mean  of  these  two  very  close  determinations  is  781*8 
foot-pounds. 

By  this  time  the  labors  of  the  previous  ten  years  had 
made  Mr.  Joule  completely  master  of  the  conditions  essen- 
tial to  accuracy  and  success.  Bringing  his  ripened  experi- 
ence to  bear  upon  the  subject,  he  executed,  in  1849,  a 
series  of  40  experiments  on  the  friction  of  water,  50  ex- 
periments on  the  friction  of  mercury,  and  20  experiments 
on  the  friction  of  plates  of  cast  iron.  He  deduced  from 
these  experiments  our  present  mechanical  equivalent  of 
heat,  justly  recognized  all  over  the  world  as  "Joule's 
equivalent. ' ' 

There  are  labors  so  great  and  so  pregnant  in  conse- 
quences that  they  are  most  highly  praised  when  they  are 
most  simply  stated.  Such  are  the  labors  of  Mr.  Joule. 
They  constitute  the  experimental  foundation  of  a  princi- 
ple of  incalculable  moment,  not  only  to  the  practice,  but 
still  more  to  the  philosophy  of  Science.  Since  the  days 
of  Newton,  nothing  more  important  than  the  theory,  of 
which  Mr.  Joule  is  the  experimental  demonstrator,  has 
been  enunciated. 

I  have  omitted  all  reference  to  the  numerous  minor 
papers  with  which  Mr.  Joule  has  enriched  scientific  liter- 
ature.    Nor  have  I  alluded  to  the  important  investigations 


450  FRAGMENTS    OF   SCIENCE 

which  lie  has  conducted  jointly  with  Sir  William  Thom- 
son. But  sufficient,  I  think,  has  been  here  said  to  show 
that,  in  conferring  upon  Mr.  Joule  the  highest  honor  of 
the  Kojal  Society,  the  Council  paid  to  genius  not  only  a 
well-won  tribute,  but  one  which  had  been  fairly  earned 
twenty  years  previously.' 

'  Lord  Beaconsfield  has  recently  honored  himself  and  England  by  bestowing 
an  annual  pension  of  2001.  on  Dr.  Joule. 


XX 


THE    COPLEY    MEDALIST    OF    1871 

DR.  JULIUS  ROBERT  MAYER  was  educated  for 
the  medical  profession.  In  the  summer  of  1840, 
as  he  himself  informs  us,  he  was  at  Java,  and 
there  observed  that  the  venous  blood  of  some  of  his  pa- 
tients had  a  singularly  bright  red  color.  The  observation 
riveted  his  attention;  he  reasoned  upon  it,  and  came  to 
the  conclusion  that  the  brightness  of  the  color  was  due  to 
the  fact  that  a  less  amount  of  oxidation  sufficed  to  keep 
up  the  temperature  of  the  body  in  a  hot  climate  than  in  a 
cold  one.  The  darkness  of  the  venous  blood  he  regarded 
as  the  visible  sign  of  the  energy  of  the  oxidation. 

It  would  be  trivial  to  remark  that  accidents  such  as 
this,  appealing  to  minds  prepared  for  them,  have  often  led 
to  great  discoveries.  Mayer's  attention  was  thereby  drawn 
to  the  whole  question  of  animal  heat.  Lavoisier  had  as- 
cribed this  heat  to  the  oxidation  of  the  food.  *'One  great 
principle,"  says  Mayer,  *'of  the  physiological  theory  of 
combustion,  is  that  under  all  circumstances  the  same 
amount  of  fuel  yields,  by  its  perfect  combustion,  the 
same  amount  of  heat;  that  this  law  holds  good  even  for 
vital  processes;  and  that  hence  the  living  body,  notwith- 
standing all  its  enigmas  and  wonders,  is  incompetent  to 
generate  heat  out  of  nothing." 

But  beyond  the  power  of  generating  internal  heat,  the 

(451) 


452  FRAGMENTS   OF  SCIENCE 

animal  organism  can  also  generate  heat  outside  of  itself. 
A  blacksmith,  for  example,  by  hammering  can  heat  a  nail, 
and  a  savage  by  friction  can  warm  wood  to  its  point  of 
ignition.  Now,  unless  we  give  up  the  physiological  axiom 
that  the  living  body  cannot  create  heat  out  of  nothing, 
"we  are  driven,"  says  Mayer,  **to  the  conclusion  that  it 
is  the  total  heat  generated  within  and  without  that  is  to  be 
regarded  as  the  true  calorific  effect  of  the  matter  oxidized 
in  the  body." 

From  this,  again,  he  inferred  that  the  heat  generated 
externally  must  stand  in  a  fixed  relation  to  the  work  ex- 
pended in  its  production.  For,  supposing  the  organio 
processes  to  remain  the  same;  if  it  were  possible,  by  the 
mere  alteration  of  the  apparatus,  to  generate  different 
aftiounts  of  heat  by  the  same  amount  of  work,  it  would 
follow  that  the  oxidation  of  the  same  amount  of  material 
would  sometimes  yield  a  less,  sometimes  a  greater,  quan- 
tity of  heat.  ** Hence,"  says  Mayer,  **that  a  fixed  relation 
subsists  between  heat  and  work  is  a  postulate  of  the 
physiological   theory   of   combustion." 

This  is  the  simple  and  natural  account,  given  subse- 
quently by  Mayer  himself,  of  the  course  of  thought  started 
by  his  observation  in  Java.  But  the  conviction  once 
formed,  that  an  unalterable  relation  subsists  between  work 
and  heat,  it  was  inevitable  that  Mayer  should  seek  to  ex- 
press it  numerically.  It  was  also  inevitable  that  a  mind 
like  his,  having  raised  itself  to  clearness  on  this  important 
point,  should  push  forward  to  consider  the  relationship  of 
natural  forces  generally.  At  the  beginning  of  1842  his 
work  had  made  considerable  progress;  but  he  had  become 
physician  to  the  town  of  Heilbronn,  and  the  duties  of  his 
profession   limited   the    time   which   he    could   devote   to 


THE   COPLEY  MEDALIST   OF  1871  453 

purely  scientific  inquiry.  He  thouglit  it  wise,  therefore, 
to  secure  himself  against  accident,  and  in  the  spring  of 
1842  wrote  to  Liebig,  asking  him  to  publish  in  his  "An- 
nalen"  a  brief  preliminary  notice  of  the  work  then  accom- 
plished. Liebig  did  so,  and  Dr.  Mayer's  first  paper  is 
contained  in  the  May  number  of  the  "Annalen'*  for  1842. 
Mayer  had  reached  his  conclusions  by  reflecting  on  the 
complex  processes  of  the  living  body;  but  his  first  step 
in  public  was  to  state  definitely  the  physical  principles  on 
which  his  physiological  deductions  were  to  rest.  He  be- 
gins, therefore,  with  the  forces  of  inorganic  nature.  He 
finds  in  the  universe  two  systems  of  causes  which  are  not 
mutually  convertible — the  different  kinds  of  matter  and 
the  different  forms  of  force.  The  first  quality  of  both 
he  affirms  to  be  indestructibility.  A  force  cannot  become 
nothing,  nor  can  it  arise  from  nothing.  Forces  are  con- 
vertible, but  not  destructible.  In  the  terminology  of  his 
time,  he  then  gives  clear  expression  to  the  ideas  of  poten- 
tial and  dynamic  energy,  illustrating  his  point  by  a  weight 
resting  upon  the  earth,  suspended  at  a  height  above  the 
earth,  and  actually  falling  to  the  earth.  He  next  fixes  his 
attention  on  cases  where  motion  is  apparently  destroyed, 
without  producing  other  motion;  on  the  shock  of  inelastic 
bodies  for  example.  Under  what  form  does  the  vanished 
motion  maintain  itself?  Experiment  alone,  says  Mayer, 
can  help  us  here.  He  warms  water  by  stirring  it;  he  re- 
fers to  the  force  expended  in  overcoming  friction.  Motion 
in  both  cases  disappears;  but  heat  is  generated,  and  the 
quantity  generated  is  the  equivalent  of  the  motion  de> 
stroyed.  "Our  locomotives,'*  he  observes  with  extraor- 
dinary sagacity,  "maybe  compared  to  distilling  apparatus: 
the  heat  beneath  the  boiler  passes  into  the  motion  of  tha 


454  FRAGMENTS    OF  SCIENCE 

train,  and  is  again  deposited  as  lieat  in  the  axles  and 
wheels." 

A  mimerical  solution  of  the  relation  between  heat  and 
work  was  what  Mayer  aimed  at,  and  toward  the  end  of 
his  first  paper  he  makes  the  attempt.  It  was  known  that 
a  definite  amount  of  air,  in  rising  one  degree  in  tempera- 
ture, can  take  up  two  different  amounts  of  heat.  If  its 
volume  be  kept  constant,  it  takes  up  one  amount:  if  its 
pressure  be  kept  constant  it  takes  up  a  different  amount. 
These  two  amounts  are  called  the  specific  heat  under  con- 
stant volume  and  under  constant  pressure.  The  ratio  of 
the  first  to  the  second  is  as  1  :  1*421.  Ko  man,  to  my 
knowledge,  prior  to  Dr.  Mayer,  penetrated  the  significance 
of  these  two  numbers.  He  first  saw  that  the  excess  0  421 
was  not,  as  then  universally  supposed,  heat  actually  lodged 
in  the  gas,  but  heat  which  had  been  actually  consumed  by 
the  gas  in  expanding  against  pressure.  The  amount  of 
work  here  performed  was  accurately  known,  the  amount 
of  heat  consumed  was  also  accurately  known,  and  from 
these  data  Mayer  determined  the  mechanical  equivalent 
of  heat.  Even  in  this  first  paper  he  is  able  to  direct 
attention  to  the  enormous  discrepancy  between  the  theo- 
retic power  of  the  fuel  consumed  in  steam-engines  and 
their  useful  effect. 

Though  this  paper  contains  but  the  germ  of  his  furthei 
labors,  I  think  it  may  be  safely  assumed  that,  as  regards 
the  mechanical  theory  of  heat,  this  obscure  Heilbronn 
physician,  in  the  year  1842,  was  in  advance  of  all  the 
scientific  men  of  the  time. 

'  Having,  by  the  publication  of  this  paper,  secured  him* 
self  against  what  he  calls  "  E ventualitaten,  * '  he  devoted 
every  hour  of  his  spare  time  to  his  studies,  and,  in  1845, 


THE   COPLEY  MEDALIST   OF   1871  455 

published  a  memoir  which  far  transcends  his  first  one  in 
weight  and  fulness,  and,  indeed,  marks  an  epoch  in  the 
history  of  science.  The  title  of  Mayer's  first  paper  was, 
*' Remarks  on  the  Forces  of  Inorganic  Nature."  The  title 
of  his  second  great  essay  was,  **  Organic  Motion  in  its 
Connection  with  Nutrition."  In  it  he  expands  and  illus- 
trates the  physical  principles  laid  down  in  his  first  brief 
paper.  He  goes  fully  through  the  calculation  of  the  me- 
chanical equivalent  of  heat.  He  calculates  the  perform- 
ances of  steam-engines,  and  finds  that  100  lbs.  of  coal,  in 
a  good  working  engine,  produce  only  the  same  amount  of 
heat  as  95  lbs.  in  an  unworking  one;  the  5  missing  lbs. 
having  been  converted  into  work.  He  determines  the  use- 
ful effect  of  gunpowder,  and  finds  nine  per  cent  of  the 
force  of  the  consumed  charcoal  invested  on  the  moving 
ball.  He  records  observations  on  the  heat  generated  in 
water  agitated  by  the  pulping-engine  of  a  paper  manufac- 
tory, and  calculates  the  equivalent  of  that  heat  in  horse- 
power. He  compares  chemical  combination  with  mechan- 
ical combination — the  union  of  atoms  with  the  imion  of 
falling  bodies  with  the  earth.  He  calculates  the  velocity 
with  which  a  body  starting  at  an  infinite  distance  would 
strike  the  earth's  surface,  and  finds  that  the  heat  gener- 
ated by  its  collision  would  raise  an  equal  weight  of  water 
17,356°  C.  in  temperature.  He  then  determines  the  ther- 
mal effect  which  would  be  produced  by  the  earth  itself 
falling  into  the  sun.  So  that  here,  in  1845,  we  have  the 
germ  of  that  meteoric  theory  of  the  sun's  heat  which 
Mayer  developed  with  such  extraordinary  ability  three 
years  afterward.  He  also  points  to  the  almost  exclusive 
efficacy  of  the  sun's  heat  in  producing  mechanical  motions 
upon  the  earth,  winding  up   with  the  profound  remark, 


456  FRAGMENTS   OF  SCIENCE 

that  the  heat  developed  bj  friction  in  the  wheels  of  our 
wind  and  water  mills  comes  from  the  sun  in  the  form  of 
vibratory  motion;  while  the  heat  produced  by  mills  driven 
by  tidal  action  is  generated  at  the  expense  of  the  earth's 
axial  rotation. 

Having  thus,  with  firm  step,  passed  through  the  powers 
of  inorganic  nature,  his  next  object  is  to  bring  his  princi- 
ples to  bear  upon  the  phenomena  of  vegetable  and  animal 
life.  Wood  and  coal  can  bum;  whence  come  their  heat, 
and  the  work  producible  by  that  heat?  From  the  im- 
measurable reservoir  of  the  sun.  Nature  has  proposed  to 
herself  the  task  of  storing  up  the  light  which  streams 
earthward  from  the  sun,  and  of  casting  into  a  permanent 
form  the  most  fugitive  of  all  powers.  To  this  end  she 
has  overspread  the  earth  with  organisms  which,  while  liv- 
ing, take  in  the  solar  light,  and  by  its  consumption  generate 
forces  of  another  kind.  These  organisms  are  plants.  The 
vegetable  world,  indeed,  constitutes  the  instrument  where- 
by the  wave- motion  of  the  sun  is  changed  into  the  rigid 
form  of  chemical  tension,  and  thus  prepared  for  future  use. 
With  this  prevision,  as  shall  subsequently  be  shown,  the 
existence  of  the  human  race  itself  is  inseparably  con- 
nected. It  is  to  be  observed  that  Mayer's  utterances  are 
far  from  being  anticipated  by  vague  statements  regarding 
the  *' stimulus"  of  light,  or  regarding  coal  as  '*  bottled  sun- 
light." He  first  saw  the  full  meaning  of  Be  Saussure^s 
observation  as  to  the  reducing  power  of  the  solar  rays, 
and  gave  that  observation  its  proper  place  in  the  doctrine 
of  conservation.  In  the  leaves  of  a  tree,  the  carbon  and 
oxygen  of  carbonic  acid,  and  the  hydrogen  and  oxygen  of 
water,  are  forced  asunder  at  the  expense  of  the  sun,  and 
the  amount  of  power  thus  sacrificed  is  accurately  restored 


THE   COPLEY  MEDALIST   OF  ISrl  457 

by  the  combustion  of  the  tree.  The  heat  and  work  poten- 
tial in  our  coal  strata  are  so  much  strength  withdrawn 
from  the  sun  of  former  ages.  Mayer  lays  the  axe  to  the 
root  of  the  notions  regarding  ** vital  force,"  which  were 
prevalent  when  he  wrote.  With  the  plain  fact  before  as 
that  in  the  absence  of  the  solar  rays  plants  cannot  per- 
form the  work  of  reduction,  or  generate  chemical  tensions, 
it  is,  he  contends,  incredible  that  these  tensions  should  be 
caused  by  the  mystic  play  of  the  vital  force.  Such  a  hy- 
pothesis would  cut  off  all  investigation ;  it  would  land  us  in 
a  chaos  of  unbridled  fantasy.  *'I  count,"  he  says,  "there- 
fore, upon  your  agreement  with  me  when  I  state,  as  an 
axiomatic  truth,  that  during  vital  processes  the  conversion 
only,  and  never  the  creation  of  matter  or  force  occurs." 

Having  cleared  his  way  through  the  vegetable  world, 
as  he  had  previously  done  through  inorganic  nature, 
Mayer  passes  on  to  the  other  organic  kingdom.  The 
physical  forces  collected  by  plants  become  the  property 
of  animals.  Animals  consume  vegetables,  and  cause  them 
to  reunite  with  the  atmospheric  oxygen.  Animal  heat  is 
thus  produced;  and  not  only  animal  heat,  but  animal  mo- 
tion. There  is  no  indistinctness  about  Mayer  here;  he 
grasps  his  subject  in  all  its  details,  and  reduces  to  figures 
the  concomitants  of  muscular  action.  A  bowler  who  im- 
parts to  an  8-lb.  ball  a  velocity  of  80  feet,  consumes  in 
the  act  1^  of  a  grain  of  carbon.  A  man  weighing  150 
lbs.,  who  lifts  his  own  body  to  a  height  of  8  feet,  con- 
sumes in  the  act  1  grain  of  carbon.  In  climbing  a  moun- 
tain 10,000  feet  high,  the  consumption  of  the  same  man 
would  be  2  oz.  4  drs.  50  grs.  of  carbon.  Boussingault 
had  determined  experimentally   the   addition   to  be  mad© 

to  the  food  of  horses  when  actively  working,  and  Liebig 

Science 20 


458  FRAGMENTS   OF  SCIENCE 

had  determined  the  addition  to  be  made  to  the  food  of 
men.  Employing  the  mechanical  equivalent  of  heat, 
which  he  had  previously  calculated,  Mayer  proves  the 
additional  food  to  be  amply  sufficient  to  cover  the  in- 
creased oxidation. 

But  he  does  not  content  himself  with  showing,  in  a 
general  way,  that  the  human  body  burns  according  to 
definite  laws,  when  it  performs  mechanical  work.  He 
seeks  to  determine  the  particular  portion  of  the  body 
consumed,  and  in  doing  so  executes  some  noteworthy  cal- 
culations. The  muscles  of  a  laborer  150  lbs.  in  weight 
weigh  64  lbs. ;  but  when  perfectly  desiccated  they  fall  to 
15  lbs.  Were  the  oxidation  corresponding  to  that  labor- 
er's work  exerted  on  the  muscles  alone,  they  would  be 
utterly  consumed  in  80  days.  The  heart  furnishes  a  still 
more  striking  example.  Were  the  oxidation  necessary  to 
sustain  the  heart's  action  exerted  upon  its  own  tissue,  it 
would  be  utterly  consumed  in  8  days.  And  if  we  confine 
our  attention  to  the  two  ventricles,  their  action  would  be 
sufficient  to  consume  the  associated  muscular  tissue  in  83^ 
days.  Here,  in  his  own  words,  emphasized  in  his  own 
way,  is  Mayer's  pregnant  conclusion  from  these  calcula- 
tions: *'The  muscle  is  only  the  apparatus  by  means  of 
which  the  conversion  of  the  force  is  effected;  but  it  is  not 
the  substance  consumed  in  the  production  of  the  mechanical 
effect/^  He  calls  the  blood  "the  oil  of  the  lamp  of  life"; 
it  is  the  slow-burning  fluid  whose  chemical  force,  in  the 
furnace  of  the  capillaries,  is  sacrificed  to  produce  animal 
motion.  This  was  Mayer's  conclusion  twenty-six  years 
ago.  It  was  in  complete  opposition  to  the  scientific  con- 
clusions of  his  time;  but  eminent  investigators  have  since 
amply  verified  it. 


THE   COPLEY   MEDALIST   OF   1871  459 

Thus,  in  baldest  outline,  I  have  sought  to  give  some 
notion  of  the  first  half  of  this  marvellous  essay.  The 
second  half  is  so  exclusively  physiological  that  I  do  not 
wish  to  meddle  with  it.  I  will  only  add  the  illustration 
employed  by  Mayer  to  explain  the  action  of  the  nerves 
upon  the  muscles.  As  an  engineer,  by  the  motion  of  his 
finger  in  opening  a  valve  or  loosing  a  detent,  can  liberate 
an  amount  of  mechanical  motion  almost  infinite  compared 
with  its  exciting  cause,  so  the  nerves,  acting  upon  the 
muscles,  can  unlock  an  amount  of  activity  wholly  out  of 
proportion  to  the  work  done  by  the  nerves  themselves. 

As  regards  these  questions  of  weightiest  import  to  the 
science  of  physiology,  Dr.  Mayer,  in  1845,  was  assuredly 
far  in  advance  of  all  living  men. 

Mayer  grasped  the  mechanical  theory  of  heat  with  com- 
manding .power,  illustrating  it  and  applying  it  in  the  most 
diverse  domains.  He  began,  as  we  have  seen,  with  phys- 
ical principles;  he  determined  the  numerical  relation  be- 
tween heat  and  work;  he  revealed  the  source  of  the  ener- 
gies of  the  vegetable  world,  and  showed  the  relationship 
of  the  heat  of  our  fires  to  solar  heat.  He  followed  the 
energies  which  were  potential  in  the  vegetable  up  to  their 
local  exhaustion  in  the  animal.  But,  in  1845,  a  new 
thought  was  forced  upon  him  by  his  calculations.  He 
then,  for  the  first  time,  drew  attention  to  the  astounding 
amount  of  heat  generated  by  gravity  where  the  force  has 
sufficient  distance  tc  act  through.  He  proved,  as  I  have 
before  stated,  the  heat  of  collision  of  a  body  falling  from 
an  infinite  distance  to  the  earth  to  be  sufficient  to  raise 
the  temperature  of  a  quantity  of  water,  equal  to  the  fall- 
ing body  in  weight,  17,356°  C.  He  also  found,  in  1845, 
that  the  gravitating  force  between  the  earth  and  sun  was 


460  FRAGMENTS    OF   SCIENCE 

competent  to  generate  an  amount  of  heat  equal  to  that 
obtainable  from  the  combustion  of  6,000  times  the  weight 
of  the  earth  of  solid  coal.  With  the  quickness  of  genius 
he  saw  that  we  had  here  a  power  sufficient  to  produce 
the  enormous  temperature  of  the  sun,  and  also  to  account 
for  the  primal  molten  condition  of  our  own  planet.  Mayer 
shows  the  utter  inadequacy  of  chemical  forces,  as  we 
know  them,  to  produce  or  maintain  the  solar  temperature. 
He  shows  that  were  the  sun  a  lump  of  coal  it  would  be 
utterly  consumed  in  5,000  years.  He  shows  the  difficul- 
ties attending  the  assumption  that  the  sun  is  a  cooling 
body;  for,  supposing  it  to  possess  even  the  high  specific 
heat  of  water,  its  temperature  would  fall  15,000°  in  5,000 
years.  He  finally  concludes  that  the  light  and  heat  of  the 
sun  are  maintained  by  the  constant  impact  of  meteoric 
matter.  I  never  ventured  an  opinion  as  to  the  truth  of 
this  theory;  that  is  a  question  which  may  still  have  to 
be  fought  out.  But  I  refer  to  it  as  an  illustration  of  the 
force  of  genius  with  which  Mayer  followed  the  mechan- 
iqal  theory  of  heat  through  all  its  applications.  "Whether 
the  meteoric  theory  be  a  matter  of  fact  or  not,  with  him 
abides  the  honor  of  proving  to  demonstration  that  the 
light  and  heat  of  suns  and  stars  may  be  originated  and 
maintained  by  the  collisions  of  cold  planetary  matter. 

It  is  the  man  who  with  the  scantiest  data  could  ac- 
complish all  this  in  six  short  years,  and  in  the  hours 
snatched  from  the  duties  of  an  arduous  profession,  that 
the  Koyal  Society,  in  1871,  crowned  with  its  highest 
honor. 

Comparing  this  brief  history  with  that  of  the  Copley 
Medalist  of  1870,  the  differentiating  influence  of  "envi- 
ronment," on  two  nvinds  of  similar  natural  cast  and  en- 


THE   COPLEY  MEDALIST   OF  1871  461 

dowment,  comes  out  in  an  instructive  manner.  With- 
drawn from  mechanical  appliances,  Mayer  fell  back  upon 
reflection,  selecting  with  marvellous  sagacity,  from  exist- 
ing physical  data,  the  single  result  on  which  could  be 
founded  a  calculation  of  the  mechanical  equivalent  of 
heat.  In  the  midst  of  mechanical  appliances,  Joule  re- 
sorted to  experiment,  and  laid  the  broad  and  firm  founda- 
tion which  has  secured  for  the  mechanical  theory  the  ac- 
ceptance it  now  enjoys.  A  great  portion  of  Joule's  time 
was  occupied  in  actual  manipulation;  freed  from  this, 
Mayer  had  time  to  follow  the  theory  into  its  most  ab- 
struse and  impressive  applications.  With  their  places  re- 
versed, however,  Joule  might  have  become  Mayer,  and 
Mayer  might  have  become  Joule. 

It  does  not  lie  within  the  scope  of  these  brief  articles 
to  enter  upon  the  developments  of  the  Dynamical  Theory 
accomplished  since  Joule  and  Mayer  executed  their  mem- 
orable labors. 


XXI 

DEATH    BY    LIGHTNING 

PEOPLE  in  general  imagine,  when  they  think  at  all 
about  the  matter,  that  an  impression  upon  the 
nerves — a  blow,  for  example,  or  the  prick  of  a 
pin — is  felt  at  the  moment  it  is  inflicted.  But  this  is 
not  the  case.  The  seat  of  sensation  being  the  brain,  to  it 
the  intelligence  of  any  impression  made  upon  the  nerves 
has  to  be  transmitted  before  this  impression  can  become 
manifest  as  consciousness.  The  transmission,  moreover, 
requires  time^  and  the  consequence  is,  that  a  wound  in- 
flicted on  a  portion  of  the  body  distant  from  the  brain  is 
more  tardily  appreciated  than  one  inflicted  adjacent  to  the 
brain.  By  an  extremely  ingenious  experimental  arrange- 
ment, Helmholtz  has  determined  the  velocity  of  this  ner- 
vous transmission,  and  finds  it  to  be  about  eighty  feet  a 
second,  or  less  than  one -thirteenth  of  the  velocity  of 
sound  in  air.  If,  therefore,  a  whale  forty  feet  long  were 
wounded  in  the  tail,  it  would  not  be  conscious  of  the  in- 
jury till  half  a  second  after  the  wound  had  been  inflicted/ 
But  this  is  not  the  only  ingredient  in  the  delay.  There 
can  scarcely  be  a  doubt  that  to  every  act  of  consciousness 
belongs  a  determinate  molecular  arrangement  of  the  brain 

'  A  most  admirable  lecture  on  the  velocity  of  nervous  transmission  has  been 
published  by  Dr.  Du  Bois-Reymond  in  the  ''Proceedings  of  the  Royal  Institu- 
tion" for  1866,  vol.  iv.  p.  576. 

(462) 


DEATH   BY   LIGHTNING  463 

— that  every  thought  or  feeling  has  its  physical  correlative 
in  that  organ;  and  nothing  can  be  more  certain  than  that 
every  physical  change,  whether  molecular  or  mechanical, 
requires  time  for  its  accomplishment.  So  that,  besides  the 
interval  of  transmission,  a  still  further  time  is  necessary 
for  the  brain  to  put  itself  in  order — for  its  molecules  to 
take  up  the  motions  or  positions  necessary  to  the  com- 
pletion of  consciousness.  Helmholtz  considers  that  one- 
tenth  of  a  second  is  demanded  for  this  purpose.  Thus, 
in  the  case  of  the  whale  above  supposed,  we  have  first 
half  a  second  consumed  in  the  transmission  of  the  intel- 
ligence through  the  sensor  nerves  to  the  head,  one-tenth 
of  a  second  consumed  by  the  brain  in  completing  the  ar- 
rangements necessary  to  consciousness,  and,  if  the  velocity 
of  transmission  through  the  motor  be  the  same  as  that 
through  the  sensor  nerves,  half  a  second  in  sending  a 
command  to  the  tail  to  defend  itself.  Thus  one  second 
and  a  tenth  would  elapse  before  an  impression  made  upon 
its  caudal  nerves  could  be  responded  to  by  a  whale  forty 
feet  long. 

Now,  it  is  quite  conceivable  that  an  injury  might  be 
inflicted  so  rapidly  that  within  the  time  required  by  the 
brain  to  complete  the  arrangements  necessary  to  conscious- 
ness, its  power  of  arrangement  might  be  destroyed.  In 
such  a  case,  though  the  injury  might  be  of  a  nature  to 
cause  death,  this  would  occur  without  pain.  Death  in 
this  case  would  be  simply  the  sudden  negation  of  life, 
without   any   intervention   of   consciousness   whatever. 

The  time  required  for  a  rifle-bullet  to  pass  clean 
through  a  man's  head  may  be  roughly  estimated  at  a 
thousandth  of  a  second.  Here,  therefore,  we  should  have 
no  room  for  sensation,  and  death  would  be  painless.     But 


464  FRAGMENTS    OF  SCIENCE 

there  are  other  actions  which  far  transcend  in  rapidity 
that  of  the  rifle -bullet.  A  flash  of  lightning  cleaves  a 
cloud,  appearing  and  disappearing  in  less  than  a  hundred- 
thousandth  of  a  second,  and  the  velocity  of  electricity  is 
such  as  would  carry  it  in  a  single  second  over  a  distance 
almost  equal  to  that  which  separates  the  earth  and  moon. 
It  is  well  known  that  a  luminous  impression  once  made 
upon  the  retina  endures  for  about  one-sixth  of  a  second, 
and  that  this  is  the  reason  why  we  see  a  continuous  band 
of  light  when  a  glowing  coal  is  caused  to  pass  rapidly 
through  the  air.  A  body  illuminated  by  an  instantaneous 
flash  continues  to  be  seen  for  the  sixth  of  a  second  after 
the  flash  has  become  extinct;  and  if  the  body  thus  illu- 
minated be  in  motion,  it  appears  at  rest  at  the  place 
where  the  flash  falls  upon  it.  When  a  color-top  with 
differently-colored  sectors  is  caused  to  spin  rapidly  the 
colors  blend  together.  Such  a  top,  rotating  in  a  dark 
room  and  illuminated  by  an  electric  spark,  appears  mo- 
tionless, each  distinct  color  being  clearly  seen.  Professor 
Dove  has  found  that  a  flash  of  lightning  produces  the 
same  effect.  During  a  thunderstorm  he  put  a  color-top 
in  exceedingly  rapid  motion,  and  found  that  every  flash 
revealed  the  top  as  a  motionless  object  with  its  colors  dis- 
tinct. If  illuminated  solely  by  a  flash  of  lightning,  the 
motion  of  all  bodies  on  the  earth's  surface  would,  as 
Dove  has  remarked,  appear  suspended.  A  cannon-ball, 
for  example,  would  have  its  flight  apparently  arrested, 
and  would  seem  to  hang  motionless  in  space  as  long  as 
the  luminous  impression  which  revealed  the  ball  remained 
upon  the  eye. 

If,  then,  a  rifle- bullet  move  with  sufficient  rapidity  to 
destroy  life  without  the  interposition  of   sensation,  much 


DEATH  BY  LIGHTNING  465 

more  is  a  flash  of  lightning  competent  to  produce  this 
effect.  Accordingly,  we  have  well-authenticated  cases  of 
people  being  struck  senseless  by  lightning  who,  on  re- 
covery, had  no  memory  of  pain.  The  following  circum- 
stantial case  is  described  by  Hemmer: 

On  June  80,  1788,  a  soldier  in  the  neighborhood  of 
Mannheim,  being  overtaken  by  rain,  placed  himself  under 
a  tree,  beneath  which  a  woman  had  previously  taken  shel- 
ter. He  looked  upward  to  see  whether  the  branches  were 
thick  enough  to  afford  the  required  protection,  and,  in 
doing  so,  was  struck  by  lightning,  and  fell  senseless  to 
the  earth.  The  woman  at  his  side  experienced  the  shock 
in  her  foot,  but  was  not  struck  down.  Some  hours  after- 
ward the  man  revived,  but  remembered  nothing  about 
what  had  occurred,  save  the  fact  of  his  looking  up  at 
the  branches.  This  was  his  last  act  of  consciousness,  and 
he  passed  from  the  conscious  to  the  unconscious  condition 
without  pain.  The  visible  marks  of  a  lightning  stroke 
are  usually  insignificant:  the  hair  is  sometimes  burned; 
slight  wounds  are  observed;  while,  in  some  instances, 
a  red  streak  marks  the  track  of  the  discharge  over  the 
skin. 

Under  ordinary  circumstances,  the  discharge  from  a 
small  Leyden  jar  is  exceedingly  unpleasant  to  me.  Some 
time  ago  I  happened  to  stand  in  the  presence  of  a  nu- 
merous audience,  with  a  battery  of  fifteen  large  Leyden 
jars  charged  beside  me.  Through  some  awkwardness  on 
my  part,  I  touched  a  wire  leading  from  the  battery,  and 
the  discharge  went  through  my  body.  Life  was  abso- 
lutely blotted  out  for  a  very  sensible  interval,  without  a 
trace  of  pain.  In  a  second  or  so  consciousness  returned; 
I  vaguely  discerned  the  audience  and  apparatus,  and,  by 


466  FRAGMENTS    OF  SCIENCE 

the  help  of  these  external  appearances,  immediately  con- 
eluded  that  I  had  received  the  battery  discharge.  The 
intellectual  consciousness  of  my  position  was  restored  with 
exceeding  rapidity,  but  not  so  the  optical  consciousness. 
To  prevent  the  audience  from  being  alarmed,  I  observed 
that  it  had  often  been  my  desire  to  receive  accidentally 
such  a  shock,  and  that  my  wish  had  at  length  been  ful- 
filled. But,  while  making  this  remark,  the  appearance 
which  my  body  presented  to  my  eyes  was  that  of  a  num- 
ber of  separate  pieces.  The  arms,  for  example,  were  de- 
tached from  the  trunk,  and  seemed  suspended  in  the  air. 
In  fact,  memory  and  the  power  of  reasoning  appeared  to 
be  complete  long  before  the  optic  nerve  was  restored  to 
healthy  action.  But  what  I  wish  chiefly  to  dwell  upon 
here  is,  the  absolute  painlessness  of  the  shock;  and  there 
cannot,  I  think,  be  a  doubt  that,  to  a  person  struck  dead 
by  lightning,  the  passage  from  life  to  death  occurs  without 
consciousness  being  in  the  least  degree  implicated.  It  is 
an  abrupt  stoppage  of  sensation,  unaccompanied  by  a  pang. 


XXII 

SCIENCE    AND    THE    "SPIRITS" 

THEIK  refusal  to  investigate  "spiritual  phenomena" 
is  often  urged  as  a  reproach  against  scientific  men. 
I  here  propose  to  give  a  sketch  of  an  attempt  to 
apply  to  the  "phenomena"  those  methods  of  inquiry  which 
are  found  available  in  dealing  with  natural  truth. 

Some  years  ago,  when  the  spirits  were  particularly 
active  in  this  country,  Faraday  was  invited,  or  rather 
entreated,  by  one  of  his  friends  to  meet  and  question 
them.  He  had,  however,  already  made  their  acquaint- 
ance, and  did  not  wish  to  renew  it.  I  had  not  been  so 
privileged,  and  he  therefore  kindly  arranged  a  transfer 
of  the  invitation  to  me.  The  spirits  themselves  named 
the  time  of  meeting,  and  I  was  conducted  to  the  place 
at  the  day  and  hour  appointed. 

Absolute  unbelief  in  the  facts  was  by  no  means  my 
condition  of  mind.  On  the  contrary,  I  thought  it  prob- 
able that  some  physical  principle,  not  evident  to  the  spir- 
itualists themselves,  might  underlie  their  manifestations. 
Extraordinary  effects  are  produced  by  the  accumulation 
of  small  impulses.  Galileo  set  a  heavy  pendulum  in  mo- 
tion by  the  well-timed  puffs  of  his  breath.  Ellicott  set 
one  clock  going  by  the  ticks  of  another,  even  when  the 
two  clocks  were  separated  by  a  wall.  Preconceived  no- 
tions  can,   moreover,  vitiate,  to   an   extraordinary  degree, 

(467) 


468  FRAGMENTS    OF   SCIENCE 

the  testimony  of  even  veracious  persons.  Hence  my  de- 
sire to  witness  those  extraordinary  phenomena,  the  exist- 
ence of  which  seemed  placed  beyond  a  doubt  by  the 
known  veracity  of  those  who  had  witnessed  and  described 
them.  The  meeting  took  place  at  a  private  residence  in 
the  neighborhood  of  London.  My  host,  his  intelligent 
wife,  and  a  gentleman  who  may  be  called  X.,  were  in  the 
house  when  I  arrived.  I  was  informed  that  the  '* medium" 
had  not  yet  made  her  appearance;  that  she  was  sensitive, 
and  might  resent  suspicion.  It  was  therefore  requested 
that  the  tables  and  chairs  should  be  examined  before  her 
arrival,  in  order  to  be  assured  that  there  was  no  trickery 
in  the  furniture.  This  was  done ;  and  I  then  first  learned 
that  my  hospitable  host  had  arranged  that  the  seance 
should  be  a  dinner-party.  This  was  to  me  an  unusual 
form  of  investigation;  but  I  accepted  it  as  one  of  the 
accidents  of  the  occasion. 

The  **  medium"  arrived — a  delicate -looking  young  lady, 
who  appeared  to  have  suffered  much  from  ill -health.  I 
took  her  to  dinner  and  sat  close  beside  her.  Facts  were 
absent  for  a  considerable  time,  a  series  of  very  wonderful 
narratives  supplying  their  place.  The  duty  of  belief  on 
the  testimony  of  witnesses  was  frequently  insisted  on.  X. 
appeared  to  be  a  chosen  spiritual  agent,  and  told  us  many 
surprising  things.  He  affirmed  that,  when  he  took  a  pen 
in  his  hand,  an  influence  ran  from  his  shoulder  downward, 
and  impelled  him  to  write  oracular  sentences.  I  listened 
for  a  time,  offering  no  observation.  *'And  now,"  contin- 
ued X.,  **this  power  has  so  risen  as  to  reveal  to  me  the 
thoughts  of  others.  Only  this  morning  I  told  a  friend 
what  he  was  thinking  of,  and  what  he  intended  to  do  dur- 
ing the  day."     Here,  I  thought,  is  something  that  can  be  at 


SCIENCE  AND    THE    ''SPIRITS"  469 

once  tested.  I  said  immediately  to  X. :  "If  you  wish  to  win 
to  your  cause  an  apostle,  who  will  proclaim  your  principles  to 
the  world  from  the  housetop,  tell  me  what  I  am  now  think- 
ing of.**     X.  reddened,  and  did  not  tell  me  my  thought. 

Some  time  previously  I  had  visited  Baron  Eeichenbach, 
in  Vienna,  and  I  now  asked  the  young  lady  who  sat  beside 
me  whether  she  could  see  any  of  the  curious  things  which 
he  describes — the  light  emitted  by  crystals,  for  example? 
Here  is  the  conversation  which  followed,  as  extracted  from 
my  notes,  written  on  the  day  following  the  sSance, 

Medium. — **0h,  yes;  but  I  see  light  around  all  bodies." 

/. — ^**Even  in  perfect  darkness?" 

Medium, — **Yes;  I  see  luminous  atmospheres  round  all 
people.  The  atmosphere  which  surrounds  Mr.  E.  0.  would 
fill  this  room  with  light." 

/. — "You  are  aware  of  the  effects  ascribed  by  Baron 
Eeichenbach  to  magnets?" 

Medium. — "Yes;  but  a  magnet  makes  me  terribly  ill." 

I. — "Am  I  to  understand  that,  if  this  room  were  per- 
fectly dark,  you  could  tell  whether  it  contained  a  magnet, 
without  being  informed  of  the  fact?" 

Medium. — "I  should  know  of  its  presence  on  entering 
the  room." 

7.__**How?" 

Medium. — "I  should  be  rendered  instantly  ill." 

/. — "How  do  you  feel  to-day?" 

Medium. — ^"Particularly  well;  I  have  not  been  so  well 
for  months." 

I. — "Then,  may  I  ask  you  whether  there  is,  at  the 
present  moment,  a  magnet  in  my  possession?" 

The  young  lady  looked  at  me,  blushed,  and  stammered, 
"No;   I  am  not  en  rapport  with  you." 


470  FRAGMENTS    OF  SCIENCE 

I  sat  at  her  right  hand,  and  a  left-hand  pocket,  within 
six  inches  of  her  person,  contained  a  magnet. 

Our  host  here  deprecated  discussion,  as  it  "exhausted 
the  medium."  The  wonderful  narratives  were  resumed; 
but  I  had  narratives  of  my  own  quite  as  wonderful. 
These  spirits,  indeed,  seemed  clumsy  creations,  compared 
with  those  with  which  my  own  work  had  made  me  famil- 
iar. I  therefore  began  to  match  the  wonders  related  to  me 
by  other  wonders.  A  lady  present  discoursed  on  spiritual 
atmospheres,  which  she  could  see  as  beautiful  colors  when 
she  closed  her  eyes.  I  professed  myself  able  to  see  similar 
colors,  and,  more  than  that,  to  be  able  to  see  the  interior 
of  my  own  eyes.  The  medium  affirmed  that  she  could  see 
actual  waves  of  light  coming  from  the  sun.  I  retorted 
that  men  of  science  could  tell  the  exact  number  of  waves 
emitted  in  a  second,  and  also  their  exact  length.  The 
medium  spoke  of  the  performances  of  the  spirits  on  mu- 
sical instruments.  I  said  that  such  performance  was 
gross,  in  comparison  with  a  kind  of  music  which  had 
been  discovered  some  time  previously  by  a  scientific  man. 
Standing  at  a  distance  of  twenty  feet  from  a  jet  of  gas, 
he  could  command  the  flame  to  emit  a  melodious  note; 
it  would  obey,  and  continue  its  song  for  hours.  So  loud 
was  the  music  emitted  by  the  gas -flame  that  it  might  be 
heard  by  an  assembly  of  a  thousand  people.  These  were 
acknowledged  to  be  as  great  marvels  as  any  of  those  of 
spiritdom.  The  spirits  were  then  consulted,  and  I  was 
pronounced  to  be  a  first-class  medium. 

During  this  conversation  a  low  knocking  was  heard 
from  time  to  time  under  the  table.  These,  I  was  told, 
were  the  spirits'  knocks.  I  was  informed  that  one  knock, 
in  answer  to  a  question,   meant  "No";    that  two   knocks 


SCIENCE   AND    THE    ''SPIRITS''  471 

meant  "Not  yet,"  and  that  three-  knocks  meant  "Yes." 
In  answer  to  a  question  whether  I  was  a  medium,  the 
response  was  three  brisk  and  vigorous  knocks.  I  noticed 
that  the  knocks  issued  from  a  particular  locality,  and 
therefore  requested  the  spirits  to  be  good  enough  to  an- 
swer from  another  corner  of  the  table.  They  did  not 
comply;  but  1  was  assured  that  they  would  do  it,  and 
much  more,  by  and  by.  The  knocks  continuing,  I  turned 
a  wine-glass  upside  down,  and  placed  my  ear  upon  it,  as 
upon  a  stethoscope.  The  spirits  seemed  disconcerted  by 
the  act;  they  lost  their  playfulness,  and  did  not  recover 
it  for  a  considerable  time. 

Somewhat  weary  of  the  proceedings,  I  once  threw  my- 
self back  against  my  chair  and  gazed  listlessly  out  of  the 
window.  While  thus  engaged,  the  table  was  rudely 
pushed.  Attention  was  drawn  to  the  wine,  still  oscillat- 
ing in  the  glasses,  and  I  was  asked  whether  that  was  not 
convincing.  I  readily  granted  the  fact  of  motion,  and 
began  to  feel  the  delicacy  of  my  position.  There  were 
several  pairs  of  arms  upon  the  table,  and  several  pairs 
of  legs  under  it;  but  how  was  I,  without  offence,  to  ex- 
press the  conviction  which  I  really  entertained?  To  ward 
off  the  difficulty,  I  again  turned  a  wine-glass  upside  down 
and  rested  my  ear  upon  it.  The  rim  of  the  glass  was  not 
level,  and  my  hair,  on  touching  it,  caused  it  to  vibrate 
and  produce  a  peculiar  buzzing  sound.  A  perfectly  can- 
did and  warm-hearted  old  gentleman  at  the  opposite  side 
of  the  table,  whom  I  may  call  A.,  drew  attention  to  the 
sound,  and  expressed  his  entire  belief  that  it  was  spiritual. 
I,  however,  informed  him  that  it  was  the  moving  hair 
acting  on  the  glass.  The  explanation  was  not  well  re- 
ceived; and  X.,  in  a  tone  of  severe  pleasantry,  demanded 


472  FRAGMENTS    OF  SCIENCE 

whetlier  it  was  the  hair  that  had  moved  the  table.  The 
promptness  of  mj  negative  probably  satisfied  him  that 
my  notion  was  a  very  different  one. 

The  superhuman  power  of  the  spirits  was  next  dwelt 
upon.  The  strength  of  man,  it  was  stated,  was  unavailing 
in  opposition  to  theirs.  No  human  power  could  prevent 
the  table  from  moving  when  they  pulled  it.  During  the 
evening  this  pulling  of  the  table  occurred,  or  rather  was 
attempted,  three  times.  Twice  the  table  moved  when  my 
attention  was  withdrawn  from  it;  on  a  third  occasion,  I 
tried  whether  the  act  could  be  provoked  by  an  assumed 
air  of  inattention.  Grasping  the  table  firmly  between  my 
knees,  I  threw  myself  back  in  the  cliair,  and  waited,  with 
eyes  fixed  on  vacancy,  for  the  pull.  It  came.  For  some 
seconds  it  was  pull  spirit,  hold  muscle;  the  muscle,  how- 
ever, prevailed,  and  the  table  remained  at  rest.  Up  to 
the  present  moment,  this  interesting  fact  is  known  only 
to  the  particular  spirit  in  question  and  myself. 

A  species  of  mental  scene-painting,  with  which  my 
own  pursuits  had  long  rendered  me  familiar,  was  em- 
ployed to  figure  the  changes  and  distribution  of  spiritual 
power.  The  spirits,  it  was  alleged,  were  provided  with 
atmospheres,  which  combined  with  and  interpenetrated 
each  other,  and  considerable  ingenuity  was  shown  in  dem- 
onstrating the  necessity  of  time  in  effecting  the  adjust- 
ment of  the  atmospheres.  A  rearrangement  of  our  posi- 
tions was  proposed  and  carried  out;  and  soon  afterward 
my  attention  was  drawn  to  a  scarcely  sensible  vibration 
on  the  part  of  the  table.  Several  persons  were  leaning 
on  the  table  at  the  time,  and  I  asked  permission  to  touch 
the  medium's  hand.  "Oh!  1  know  I  tremble,"  was  her 
reply.     Throwing  one  leg  across  the  other,  I  accidentally 


SCIENCE   AND    THE   ''SPIRITS''  473 

nipped  a  muscle,  and  produced  thereby  an  involuntary 
vibration  of  the  free  leg.  This  vibration,  I  knew,  must 
be  communicated  to  the  floor,  and  thence  to  the  chairs  of 
all  present.  I  therefore  intentionally  promoted  it.  My 
attention  was  promptly  drawn  to  the  motion;  and  a  gen- 
tleman beside  me,  whose  value  as  a  witness  I  was  par- 
ticularly desirous  to  test,  expressed  his  belief  that  it  was 
out  of  the  compass  of  human  power  to  produce  so  strange 
a  tremor.  "I  believe,"  he  added,  earnestly,  *'that  it  is 
entirely  the  spirits'  work."  "So  do  I,"  added,  with  heat, 
the  candid  and  warm-hearted  old  gentleman  A.  "Why, 
sir,"  he  continued,  "I  feel  them  at  this  moment  shaking 
my  chair."  I  stopped  the  motion  of  the  leg.  "Now,  sir," 
A.  exclaimed,  "they  are  gone."  I  began  again,  and  A. 
once  more  affirmed  their  presence.  I  could,  however, 
notice  that  there  were  doubters  present,  who  did  not 
quite  know  what  to  think  of  the  manifestations.  I  saw 
their  perplexity;  and,  as  there  was  sufficient  reason  to 
believe  that  the  disclosure  of  the  secret  would  simply  pro- 
voke anger,  I  kept  it  to  myself. 

Again  a  period  of  conversation  intervened,  during 
which  the  spirits  became  animated.  The  evening  was 
confessedly  a  dull  one,  but  matters  appeared  to  brighten 
toward  its  close.  The  spirits  were  requested  to  spell  the 
name  by  which  I  was  known  in  the  heavenly  world.  Our 
host  commenced  repeating  the  alphabet,  and  when  he 
reached  the  letter  "P"  a  knock  was  heard.  He  began 
again,  and  the  spirits  knocked  at  the  letter  "O."  I  was 
puzzled,  but  waited  for  the  end.  The  next  letter  knocked 
down  was  "E."  I  laughed,  and  remarked  that  the  spirits 
were  going  to  make  a  poet  of  me.  Admonished  for  my 
levity,  I  was  informed  that  the  frame  of  mind  proper  for 


474  FRAGMENTS    OF   SCIENCE 

the  occasion  ought  to  have  been  superinduced  by  a  pe- 
rusal of  the  Bible  immediately  before  the  seance.  The 
spelling,  however,  went  on,  and  sure  enough  I  came  out 
a  poet.  But  matters  did  not  end  here.  Our  host  con- 
tinued his  repetition  of  the  alphabet,  and  the  next  letter 
of  the  name  proved  to  be  "O."  Here  was  manifestly  an 
unfinished  word;  and  the  spirits  were  apparently  in  their 
most  communicative  mood.  The  knocks  came  from  under 
the  table,  but  no  person  present  evinced  the  slightest  de- 
sire to  look  under  it.  I  asked  whether  I  might  go  under- 
neath; the  permission  was  granted;  so  I  crept  under  the 
table.  Some  tittered;  but  the  candid  old  A.  exclaimed, 
"He  has  a  right  to  look  into  the  very  dregs  of  it,  to  con- 
vince himself."  Having  pretty  well  assured  myself  that 
no  sound  could  be  produced  under  the  table  without  its 
origin  being  revealed,  I  requested  our  host  to  continue 
his  questions.  He  did  so,  but  in  vain.  He  adopted  a 
tone  of  tender  entreaty;  but  the  "dear  spirits"  had  become 
dumb  dogs,  and  refused  to  be  entreated.  I  continued  un- 
der that  table  for  at  least  a  quarter  of  an  hour,  after  which, 
with  a  feeling  of  despair  as  regards  the  prospects  of  hu- 
manity never  before  experienced,  I  regained  my  chair. 
Once  there,  the  spirits  resumed  their  loquacity,  and  dubbed 
me  "Poet  of  Science." 

This,  then,  is  the  result  of  an  attempt  made  by  a  scien- 
tific man  to  look  into  these  spiritual  phenomena.  It  is 
not  encouraging;  and  for  this  reason.  The  present  pro- 
moters of  spiritual  phenomena  divide  themselves  into  two 
classes,  one  of  which  needs  no  demonstration,  while  the 
other  is  beyond  the  reach  of  proof.  The  victims  like  to 
believe,  and  they  do  not  like  to  be  undeceived.  Science 
is   perfectly  powerless    in    the    presence   of    this   frame   of 


SCIENCE   AND    THE    ''SPIRITS''  475 

mind.  It  is,  moreover,  a  state  perfectly  compatible  with 
extreme  intellectual  subtlety  and  a  capacity  for  devising 
hypotheses  which  only  require  the  hardihood  engendered 
by  strong  conviction,  or  by  callous  mendacity,  to  render 
them  impregnable. 

The  logical  feebleness  of  science  is  not  sufficiently 
borne  in  mind.  It  keeps  down  the  weed  of  superstition, 
not  by  logic  but  by  slowly  rendering  the  mental  soil 
unfit  for  its  cultivation.  When  science  appeals  to  uni- 
form experience,  the  spiritualist  will  retort,  "How  do 
you  know  that  a  uniform  experience  will  continue  uni- 
form? You  tell  me  that  the  sun  has  risen  for  six  thou- 
sand years:  that  is  no  proof  that  it  will  rise  to-morrow; 
within  the  next  twelve  hours  it  may  be  puffed  out  by  the 
Almighty."  Taking  this  ground,  a  man  may  maintain 
the  story  of  "Jack  and  the  Beanstalk"  in  the  face  of  all 
the  science  in  the  world.  You  urge,  in  vain,  that  science 
has  given  us  all  the  knowledge  of  the  universe  which  we 
now  possess,  while  spiritualism  has  added  nothing  to  that 
knowledge.  The  drugged  soul  is  beyond  the  reach  of 
reason.  It  is  in  vain  that  impostors  are  exposed,  and  the 
special  demon  cast  out.  He  has  but  slightly  to  change 
his  shape,  return  to  his  house,  and  find  it  "empty,  swept, 
and  garnished." 

Since  the  time  when  the  foregoing  remarks  were  writ- 
ten I  have  been  more  than  once  among  the  spirits,  at  their 
own  invitation.  They  do  not  improve  on  acquaintance. 
Surely  no  baser  delusion  ever  obtained  donunance  over 
the  weak  mind  of  man. 

END  OP  VOL.   I.   OP  "  FRAGMENTS  OP  SCIENCE" 


THE  UNIVERSITY  LIBRARY 

UNIVERSITY  OF  CALIFORNIA,  SANTA  CRUZ 

SCIENCE  LIBRARY 


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